Crossover/Sandbox

Cross Overs occur anytime characters from one work "cross over" into another. Any time characters from one work interact with another work, you have a Crossover. The crossover started itself off as a good way to make the best of what you've got — so if you have two shows on your roster, it's a no-brainer to have the shows and characters cross over every once in a while, especially if one of the shows is less popular than its sibling.

Now there's a whole host of subtrope of ways in which people mix characters and plots from different works of fiction for fun and profit. They are particularly important to Fanfic writers, as evidenced by Rule Fifty, since the fact that they are borrowing characters means there's no limit to just borrowing from one work.

Subtropes

 * Bat Family Crossover
 * Canon Welding
 * Character Overlap
 * Common Crossover
 * Crisis Crossover
 * Crossover
 * Cross Through
 * Crossover Punchline
 * Crossover Ship
 * Deconstruction Crossover
 * Doppelganger Crossover
 * Fake Crossover
 * Fully-Absorbed Finale
 * Fusion Fic
 * Guest Fighter
 * Intercontinuity Crossover
 * Lawyer-Friendly Cameo
 * Massive Multiplayer Crossover
 * Mega Crossover
 * Modular Franchise
 * Official Parody
 * Original Generation
 * Poorly-Disguised Pilot
 * Power Creep, Power Seep
 * Red Skies Crossover
 * Required Spinoff Crossover
 * Story-Breaker Team-Up
 * Team-Up Series
 * Transplant
 * Rogues Gallery Transplant
 * Ultimate Showdown of Ultimate Destiny

Anime and Manga

 * The different stories Nasuverse are all coliding in a hilarious new Affectionate Parody called Carnival Phantasm. The first episode featured the characters of Fate/stay night finding out that the Holy Grail War had become a game show. Numerous people from throughout the Nasuverse can be seen in the audience.
 * "Cross Epoch", a special manga where Dragon Ball and One Piece crossed over.
 * There's also an anime crossover special of Toriko and One Piece.
 * In the Manga Digimon Adventure V-tamer 01 are a total of 3 non canon crossovers, each being about an hero of the current digimon anime series being space warped and ending up battling an extremely powerful digimon together with Taichi and Zero.
 * In honor of Shounen Sunday's 50th anniversary, the publisher released a DVD including three OVAs for their three most popular animes: Inuyasha, Urusei Yatsura, and Ranma ½. The DVD also included this animated short, where the main characters of the three series met up. Hilarity Ensues.
 * Voltron: The Fleet of Doom, a TV special made for American audience which had Lion Voltron and Vehicle Voltron working together to battle the allied forces of King Zarkon and the Drule Empire.
 * Recently, Conan Edogawa and Lupin the Third met one another in a movie special.
 * One episode of Dragon Ball took place in Penguin Village.
 * And Doctor Slump's 1997 remake anime has Goku visits Penguin Village. The different is that Arale is now a main character while Goku is a guest star, and Art Evolution.
 * In one episode of the 1980s Astro Boy, the titular robot is taken back in time, along with his sister Uran, the one and only Black Jack and the doctor's assistant/companion Pinoko, in order to save the main character from Princess Knight. Good ol' Tezuka mix-and-match.
 * Leiji Matsumoto's works. Like with CLAMP everything is in the same universe (Yamato and Harlock and Maetel and Emeraldas and the Queen Millenium...) but details do not match (Tenshiro died in three different ways already).

Comics
"Superman: "We're surrounded, you know. I can hear them all."
 * This was the entire premise of The League of Extraordinary Gentlemen -- graphic novels and movie both.
 * Archie Comics is not afraid of this. The only real thing that defines the universes (excluding Sonic the Hedgehog and other licensed series) is different towns in America. The Archies performing alongside Josie and the Pussy Cats? Sure!
 * Sabrina however did crossover with Sonic the Hedgehog, in a two part-special that expanded both series.
 * Archie Meets The Punisher. Yes, really.
 * Tiny Titans meets Little Archie. Yes, really.
 * In fairness, the early 90's Frank Castle was essentially a well-equipped assassin, not an amoral wholesale slaughtering machine leaving dozens of bloody corpses everywhere. And he not only engages in less violence here than his usual missions of the time, he even lets his target live. Still, the fact that everyone involved managed to pull it off at all, much less make a pretty good story, is downright amazing.
 * There was also a crossover of Archie and the Teenage Mutant Ninja Turtles. Here is a review of it, courtesy of Linkara.
 * The "Night At the Comic Book Shop" trade paperback had Archie deal with some old comic characters that Archie Comics punished.
 * The comic book version of Archie's Weird Mysteries used a dimensional warp or some such to finally let Archie meet the Mighty Crusaders, Archie Comics's rarely-used super hero team.
 * José Carioca's universe is set in his home country of Brazil, but he's shown to be friends with Donald Duck in Saludos Amigos and The Three Caballeros (as well as the ride in the Mexican pavilion at Epcot). Thus, there are occasional crossovers between the Carioca universe and the McDuck Universe. Indeed, there has been a prolonged arc of stories where José visits Duckburg, and interacts with the various characters there.
 * Various Image Comics titles (Invincible, Savage Dragon, Witchblade, etc.) occasionally cross over, but the editorial policy is that what happens in one title doesn't have to affect events in another. Basically each hero lives in his or her own unique universe that may or may not contain versions of the other heroes. For instance, Invincible's universe has a Savage Dragon and a Witchblade who are more or less the same as their counterparts in their own books and have roughly the same adventures, but what happens in Invincible's book stays there.
 * The very first superhero crossover was a battle between the Human Torch and the Sub-Mariner in Marvel Mystery Comics #8-9 (1940). This is pretty much the beginning of the Marvel Universe.
 * Actually, the Shield and the Wizard at MLJ met earlier. The Wizard had learned of an impending attack on Pearl Harbor and contacted the Shield. They spent the next few issues fighting the same enemies (the Soviet/Nazi proxies called Moskovians), occasionally crossing paths for a panel or three.
 * Superman and Batman have been appearing in comics together since The Silver Age of Comic Books, and Wonder Woman joining in isn't uncommon, either. Generally they team up, though having them fight each other is always a good way to drawn in readers.

Batman: "I think we can take them. Do you think we can take them?"

Superman: "You always think we can take them."

Batman: "Yes, I do."

Superman: "Then, let's do it."

(From Superman/Batman: Public Enemies)"


 * If that counts, we may as well mention All Star Comics #3, the comic which created The DCU as a shared universe.
 * Usagi Yojimbo and Teenage Mutant Ninja Turtles had several single-issue gag crossovers in their respective early runs, which culminated, several years later, in an entire volume of Usagi which centered around a village shaman summoning the Turtles to fight invading ninjas, and the Turtles' subsequent confusion at being landed in a medieval Japan inhabited by talking animals.
 * Marvel's What If series threw in a few of these over the years, notably stories in which Conan the Barbarian fought Captain America (narrow win for Conan) and Wolverine (Conan wins the first round, Wolverine regenerates and pushes Conan through a portal to the future, where he destroys the universe by throwing a rock at Cyclops' head).
 * Another notable Marvel crossover, this one in-continuity featured perhaps the two greatest Sword and Sorcery heroes (using the term loosely) of all time; Conan and Elric. As if that wasn't epic enough, Elric's creator Michael Moorcock helped with the plot. It also introduced evil sorcerer Kulan Gath, who at various times has put Red Sonja, Spider-Man, the X-Men, The Avengers, and modern day New York City through pure hell.
 * Ape Nation. Ape Nation.
 * Marvel's Squadron Supreme limited series (already itself a psuedo-Crossover, featuring a team of Captain Ersatzes from DC Comics) also had a one-issue Crossover with Captain America.
 * Cartoon Network once made a comics story in which Space Ghost's monkey sidekick, Blip, needed to save his friends from Giant Cosmic Monkey Robot and asked Magilla Gorilla, Monkey, and other simians from CN's shows for help. Together they saved the hero and defeated cosmic monkey with a giant banana, soooo...yeah.
 * At one point, they also did an entire string of Slice of Life segments between commercial breaks where pretty much all of the characters were shown coexisting in some sort of vaguely defined urban setting.
 * G.I. Joe and Transformers. While the TV show is limited to the "Old Snake" thing and the hologram of Marissa's dad (see Western Animation below), the comics have crossed over many, many times, even to the point of very important and far-reaching events in one starting in crossovers
 * The various Marvel-DC crossovers that have been published over the years, which imply the existence of a third timeline separate from the Marvel and DC continuities in which both exist alongside each other.
 * There was also the Amalgam Universe, which consists of heroes who merge traits of a hero from each universe. Examples include Iron Man Lantern (take a guess) and Darkclaw, who was Batman and Wolverine (!!!).
 * However, the events of JLA-Avengers are taken as canon for both publishers in their maisntream universes.
 * Judge Dredd has had crossovers with Strontium Dog, Rogue Trooper, Batman, Aliens, and Predator among others.
 * Every so often, the Justice League of America and Justice Society of America team up and have a crossover. In The Bronze Age of Comic Books, it happened every year; nowadays, it's more like every three years or so.
 * In the 90's, DC and Image published a crossover between Batman and Spawn. It's best remembered for completely rejecting the usual "meet, fight, become friends" routine found in most crossover comics. The two heroes hate each other for almost the entire story. At the end, Spawn wonders aloud if they could try to be friends, and Batman responds by burying a batarang in his face.
 * That is the main premise of the IDW comic Infestation: Outbreak, where a dimensional gate (which looks suspiciously like a Stargate) unleashes hordes of zombies from another world. Unlike other zombies from this world, these are guided by a single will, the Undermind. They are also somehow capable of infecting technology and other undead creatures. When an elite vampire squad is sent to deal with the situation, one of them, Britt, gets bitten and becomes a vampire/zombie hybrid. Britt uses artillica to open four more dimensional portals, through which the zombies and infected machines pour, along with parts of Britt herself. While the other vampires manage to shut down the portals, the threat has already spread to other worlds. These worlds are those of G.I. Joe, Star Trek, Transformers, and Ghostbusters. All these worlds now have to fight an infestation from a virulence they have not seen, infecting both man and machine.
 * A special magazine published in 2000 by Cartoon Network featured a five-page comic story in which Scooby Doo meets The Powerpuff Girls. Bubbles, as expected, glomps Scooby ("Puppy!!").
 * Some crossovers Linkara has reviewed include Star Trek and X-Men, Eminem and The Punisher, and Superman and the Quik Bunny.
 * Spider-Man vs. Powdered Toast Man here.
 * Asterix In Belgium features a cameo appearance by Dupont and Dupondt (or Thompson and Thomson if you prefer, although they're unnamed here anyway).

Fan Works

 * Cross Overs are huge in all fanworks, one of the two or three biggest reasons to create them.
 * A favourite for many seems to be Doctor Who, due to the remarkable ease with which the Doctor can be inserted in any story just by having the TARDIS land there. One of the most famous, The Man With No Name, is a Firefly / Doctor Who crossover, which some say should be officially added to Canon.
 * Yet it VERY strangely lists one of the unofficial rules of the internet dealing with crossovers (specifically, rule (9)
 * There's also A Hero,a crossover with Puella Magi Madoka Magica which subverts most Doctor Who crossover formulas by instead inserting Dalek Sec to fix the mess.
 * Another crossover that's become increasingly popular is Super Who, which combines Doctor Who with Supernatural.
 * Fanfic wise, it's almost impossible to find a fandom that doesn't have a crossover with Harry Potter. 
 * There are crossovers of Land Before Time with Star Wars, Eragon, Mega Man, and, in one case, Halo. Amazingly, many find that the Star Wars/Land Before time crossover in question is actually good.
 * Found the cross between Harry Potter and The Land Before Time!
 * "Voldemort has found a way to go back thousands of years, to the time of the great dinosaurs"? That's not a promising opening line.
 * There's even a Land Before Time megacrossover series on FanFiction.net. Too bad it's about the same as 90% of all fanfics.
 * There is a really good one between Harry Potter and Final Fantasy VIII that starts when Harry, age 5, is magically transported to the world of FF8 and once there he becomes a fine Gunblade fighter and is somehow able to use FF8 magic and do everything one can in the game... without using Guardian Forces. Of course, he has later to go back to his own world, where an evil Dumbledore lays in waiting... See here for the prequel+ Philosopher's Stone arc, here for the Chamber of Secrets arc and here for the Prisoner of Azkaban arc. It's a pity that the third arc is not finished...
 * Well, I, personally, won't be impressed until someone makes a crossover between Avatar: The Last Airbender and Bleach. And does it well.
 * As well as I, only with Deadpool and The Order of the Stick. Yes, there is an OotS archive over at the Pit. Luckily, they are all quite good.
 * There's also a small collection of Calvin and Hobbes/Harry Potter crossovers, all involving Calvin being a wizard. Luckily this corner of fan fiction seems to be untouched by Sturgeon's Law, although that may be because they're so small in number. Give em a look. This one is particularly good so far.
 * Harry Potter in The Princess Bride, anyone? Hermione dreams the people she knows into the plot of the movie, with herself in the role of the Grandson being read the book by Dumbledore. No, really!
 * I found an interesting one the other day that involved the plot of Pretty Little Liars being re-imagined in the Potter universe. Unlike most other crossovers, the characters of both series don't actually meet. Here it is
 * There are a few fics that cross over just about all the Nicktoons. One that does this along with extreme AU along with Alternate History (Vlad is now king of an Fictional Counterpart of America!) would be Chess Piece by Neo Yi. It's filled with tons of continuity nods and has everything, from The Wild Thornberrys, Invader Zim, and Dora the Explorer to Doug, Aaahh Real Monsters, Cat Dog, and My Life as a Teenage Robot. Seriously; it is also awesome.
 * Not many Crossover fics have managed to be as awesome as Sherlock Holmes vs Edward Cullen.
 * Star Wars and Star Trek. The rivalry between their fanbases and The Ultimate Showdown of Ultimate Destiny makes it a bygone conclusion feature some, and for those to require at least one major battle between the United Federation of Planets and the Galactic Empire, but there are certain exceptions. A particularly good one can be found here, ironically written by the creator of an infamous pro-Star Wars site.
 * Another good one is Unity by notable contributor SF Debris. What is a wormhole was connected from the Delta Quadrant circa Voyager's fifth season to the Star Wars Universe circa early Return of The Jedi, (disrupting the Rebel Alliance's attempt to attack the second Death Star.) Involved every character from TNG forward moving well into the future and exploring all the potential interactions in detail.
 * To complete the trifecta of renowned Star Wars vs. Star Trek fanfics, there's StarCrossed. If there's more epic fanfiction than Star Crossed, I haven't read it.
 * While not a Fanfic, this Youtube user makes videos which present "what if" situations dealing with television logos, which also includes gems like "What if Steven Bochco collaborated with 4K!ds Entertainment?".
 * Shinji and Warhammer 40 K.
 * SORT OF. Shinji and Warhammer 40 K is less of a straight crossover, where the universes meet, and more of a fix fic where Shinji is a badass manipulator who likes 40K. Not to say there aren't crossover elements, just... it's not the straightest example of one. A better example of an Evangelion/Warhammer crossover would be Thousand Shinji.
 * Brother Bubbles. Space Marine Terminator finds himself in Rapture. Then meets a Little Sister.
 * Spider-Man has a slew of crossovers on Fanfiction.net. This one crossed with the Teenage Mutant Ninja Turtles, and this one crossed with House are quite good.
 * In Anime fanfiction (and sometimes in other media) a quite common way to establish a plot, especially in Peggy Sue fics, is a wish granted by Belldandy or her sisters. Whatever this crossover is minor or major varies between fics.
 * Raquaza Master's Pokemon Squad fanfics are supposed to combine the characters from Pokémon, Ka Blam!!, Barney and Friends (Barney is going to be one of the main antagonists), Danny Phantom, Animaniacs, Sesame Street (Elmo's going to be an upcoming villain), many video games, and much, much more.
 * The Ranma 1/2 fandom loves to create crossovers. There exists several archives devoted to them. Particularly common are crossovers with Sailor Moon which has a sub genre called the Fuku Fic that has more fan fics written for it than most anime has fan fics total.
 * Is there anyone Sonic the Hedgehog hasn't met yet? The blue hedgehog has had crossovers with Percy Jackson, Godzilla, Transformers, Halo, iCarly, Gorillaz, the Mighty Ducks and even Tengen Toppa Gurren Lagann. Are any of these even remotely connected?
 * But of course, we all know who the big winner is...
 * Takamachi Nanoha of 2814 is a Magical Girl Lyrical Nanoha Crossover with Green Lantern. Then, it started to cross over with everything by chapter 3. Written by the almost infamous Shadow Crystal Mage, so you know it's good. Now with its own page.
 * An author on F Fnet has created this a Star Wars and Halo crossover where the characters of Republic Commando (and some friends and enemies) meet Master Chief and company in the Halo universe. There is a sequel that takes place mostly in the Star Wars universe durring the events of the Original Trilogy.
 * A Dark Knight Over Sin City. It's rather obvious what it is about, going by the title.
 * Quinn in Wonderland: Quinn Fabray falls Down the Rabbit Hole and ends up in Wonderland
 * Brutal, Horrible, and What If is a series of crossovers between Axis Powers: Hetalia and the Hunger Games. Also Alternate Universe.
 * One author has even crossed over (at various times) Transformers with Knight Rider, The Real Ghostbusters, Masters of the Universe, Sailor Moon, Inuyasha, and even Judge Judy.
 * Gabriel Blessing is well known for his two Fate/stay night crossovers:
 * The Hill of Swords, a Zero no Tsukaima crossover wherein Louise summons Shirou as her familiar. This Fanfic revived the whole concept of "Let's have Louise summon a totally different person than Saito !"
 * In Flight, a Sekirei crossover where Shirou discovers his Not Quite Dead mother, Takami Sahashi, and replaces Minato as the Ashikabi of the North.
 * Divine Intervention The MacManus brothers meet the Winchester brothers. And it is COOL.
 * One of the most prevalent type of My Little Pony Friendship Is Magic fanfics, togheter with Fem Slash. The most regular kinds are those such as Better Living Through Science and Ponies (Portal) and Turnabout Storm (Ace Attorney), where another series' characters are transported to Equestria, sometimes turning into ponies in the process, and Fusion Fics such as Fallout Equestria (guess).
 * Demon of Justice is older than Fanfiction.net and crosses Gundam Wing with Oath of Swords, a high fantasy series.
 * The Cries of Haruhi Suzumiya is a crossover between The Melancholy of Haruhi Suzumiya and Higurashi no Naku Koro ni.
 * Survivability: The Bleach universe with an original plot, starring characters from different K Pop groups (Dong Bang Shin Ki, SHINee, Big Bang, Super Junior, B2ST, +other solo singers and actors).
 * Bayonsiders has War from Darksiders and Bayonetta from Bayonetta meet up, have sex, beat up angels, beat up demons, and have sex while beating up angels and demons at the same time. All while trying to stay in continuity of both games.
 * The fancomic Roommates by Ashe Rhyder and its spinoff Girls Next Door by Pika la Cynique are basically this -- a crossover between Labyrinth, Phantom of the Opera, Pirates of the Caribbean, Les Misérables and occasionally Harry Potter -- the many fandoms that appeared only once or twice are too numerous to count. Both by now run rather independently of each other, with a frequent Continuity Nod and Shout-Out to each other during their many, many storylines. And they are also awesome.
 * Decent Pokemon/Digimon crossovers are not that common in fanfiction, but of late some interesting new submissions have popped up. Sturgeon's Law may be applicable due to grammar issues, but those with decent grammar tend to be alright.
 * Even though the first story of A Sticky Situation remains solidly in the Digimon Season three universe, the second story tosses several of the characters into Star Wars: The Clone Wars due to a dimension-traveling digimon called Parallelmon (a Canon digimon, not an OC of the author). It is very well written and very entertaining.
 * Rule (9): If it exists, there is a crossover with Touhou. The series even has its own article.
 * When With Strings Attached was first conceived, it was supposed to be a Beatles/DungeonsAndDragons crossover. Luckily, the author quickly matured out of that notion, and the story turned into something far more interesting and complex, though its RPG roots are quite evident still.
 * There are a lot of fanarts involving Kaa the Snake hypnotizing various victims. Google search "Kaa" and you'll see what the deal is.
 * It's easier to count the things that Naruto hasn't had crossovers with when it's had crossovers with The Boondocks, Duke Nukem, Aqua Team Hunger Force, Mean Girls, Godzilla, Friday the 13th and probably the strangest with Married... with Children.
 * The game-idea series Crossover X practically lives on this trope. It even uses the word "crossover" in its name.

Films

 * This was the entire premise of The League of Extraordinary Gentlemen -- graphic novels and movie both.
 * Too many Godzilla movie crossovers to list as many of his adversaries (including King Kong and Mothra) had starred in their own films previously. He even had a smackdown with his own the American Godzilla (from the 1998 film) in Godzilla: Final War.
 * Monster Mash films including:
 * Frankenstein Meets the Wolf Man
 * House of Frankenstein
 * House of Dracula
 * Abbott and Costello Meet Frankenstein
 * Abbott and Costello Meet the Invisible Man
 * Abbott and Costello Meet Dr. Jekyll and Mr. Hyde
 * Abbott and Costello Meet The Mummy
 * The Monster Squad
 * Freddy vs. Jason
 * Alien vs. Predator
 * The appearance of Tony Stark in 2008's The Incredible Hulk movie suggests a Marvel Universe film crossover.
 * Marvel Films has publicly stated that it's working its way up to an Avengers movie.
 * A SHIELD agent plays a supporting role in the Iron Man film (going so far as to drop the organization's name,) and Nick Fury mentions "The Avengers Initiative" in his post-credits cameo.
 * There's two "object cameos" in Iron Man 2, that tie into the next two Avengers to get their own films.
 * The 2005 animated film The Batman vs. Dracula.
 * Possibly the largest-scale example of this was when Disney, Warner Bros, and Turner Entertainment teamed up for the 1988 hit movie Who Framed Roger Rabbit?.
 * The View Askewniverse movies are all connected, mostly by Jay and Silent Bob appearing in every movie. Aside from that, there are numerous events, elements, and secondary characters that intertwine throughout all six films. It also helps that the movies take place in the same state.
 * There is an old film, Hercules, Samson and Ulysses which features the two strong men of the bronze age coming to blows. It's also So Bad It's Good.

Literature

 * There are a large number of Sherlock Holmes Crossover pastiches, including at least three that pit the Great Detective against Dracula.
 * Neil Gaiman's Hugo-winning 2003 story A Study in Emerald places Sherlock Holmes in the world of H.P. Lovecraft.
 * The Doctor Who Virgin New Adventures novel All-Consuming Fire did the same thing in 1994.
 * Long before the Marvel comics put two Robert E. Howard barbarian heroes together, Howard himself did it: in the Bran Mak Morn story Kings of the Night, Kull of Atlantis makes a special guest appearance. He and Bran team up against the Roman Legions invading Britain.
 * The 1995 novel H: The Story of Heathcliff's Journey Back to Wuthering Heights crosses over the two Bronte sister classics Jane Eyre and Wuthering Heights.

Live Action TV

 * The "Tommy Westphall Universe" (or, alternatively, "The John Munch Principle") ties dozens of television series from the 1970's to the present through the characters of Tommy Westphall from St Elsewhere (who is revealed to have imagined the entire series in the final scene of the final episode) and John Munch, the Baltimore detective who has made crossover appearances in many television series (including the Law and Order canon, The X-Files and Homicide: Life On the Street). Thanks to Munch's crossovers and cameos, as well as shared elements and names that occur between different franchises and series, an entire theorem was developed showcasing the connections. More than 90% of all television shows are connected via crossover, including the entire All in The Family franchise, The Wire, the entire Law and Order franchise, every major medical drama made in the 90's and 00's, and many more. Here's a handy chart for those keeping track.
 * Apparently among Mr Roark's Fantasy Island magical, mystical abilities is the power to CrossOver with other AaronSpelling series.
 * Recent episodes of Law and Order Special Victims Unit and Law and Order Trial By Jury, titled "Night" and "Day," respectively, were crossovers of each other on the same night. The major characters from each series appeared on both episodes, which dealt with a rapist. SVU focused on the investigation and arrest of the rapist, while Trial by Jury focused on the trial. (There was some discontinuity here. At the end of SVU, the case was declared a slamdunk. At the beginning of TBJ, the case became a sure loser.)
 * Law And Order: SVU did this years ago with a 2-part event, "Entitled," the second half of which took place on the original Law and Order. However, this plays havoc with syndication, because the shows are rerun on different networks in America; the SVU episode leaves the viewer hanging without the second half.
 * Law and Order also crossed over with Homicide in what was originally a two-parter. While working together on a single case, the two squads had personality clashes, and normally-infallible characters faltered in unfamiliar settings. In a good move, when this episode is rerun TNT will show both the L&O and Homicide episodes back-to-back, even though they don't usually show Homicide at all. (This also happened when Channel Five in the UK - which didn't screen Homicide - ran that particular L&O episode... unfortunately this practice is the exception rather than the rule in Britain, meaning numerous crossovers of this ilk are either left hanging (like the Ally McBeal/The Practice crossover - or, more germanely to Channel Fice, the Martial Law/Walker, Texas Ranger one ) or shown waaaaaaay apart, like the Magnum, P.I./Simon and Simon one.)
 * iCarly and Victorious.
 * iCarly and Fred.
 * Community has a great one, involving Cougar Town. Abed mentions that he was allowed on the set of Cougar Town as an extra, and that episode actually exists!
 * The whole point of moving Frasier to Seattle was to prevent network pressure for guest stars from Cheers. They couldn't hold out forever, though.
 * The character of Captain Jack Harkness first appeared in Doctor Who near the end of the 2005 series. At its finale, he was left behind by the Doctor, and was next seen in 2006 starring in his own spinoff, Torchwood. Torchwood ended its first series with Jack running away to rejoin the Doctor, which occurred near the end of Doctor Who's 2007 series in the three-part finale, after which Jack headed back to his team for Torchwood's second series.
 * Jack returns for the series 4 finale, along with Ianto and Gwen. The episode crosses over even further with the return of Sarah Jane -- accompanied by her adopted son from her own spinoff series.
 * Likewise, the Doctor has had a guest appearance in The Sarah Jane Adventures, as has the Brigadier (making him the first Classic Who character to appear in a Nu-Who spinoff, but not Nu-Who itself). Another SJA crossover aired, with Eleven and Jo Jones.
 * Martial Law has the appearance of Chuck Norris' character for a couple of episodes. In fact, for that case, you have to watch the continuation of the case in Walker, Texas Ranger if you want to know how it is resolved.
 * Stargate SG-1 and Stargate Atlantis have had several instances of crossovers. The most notable are the characters Rodney McKay and Elizabeth Weir. Often times, things that happen in one series have an impact on the other, such as the recovery of a ZPM by SG-1 helping to defend Atlantis during its siege.
 * There was a dedicated episode where SG-1 GOES to Atlantis to find out the secrets of the Deus Ex Machina that'll save the galaxy, in the process both teams save the other one from disaster by the series two Big Bad.
 * Due to the extra time created by the 2008 Writer's Strike, even some Late Night Talk Shows got in on the action. While The Daily Show and The Colbert Report often have crossovers (as Colbert's character originated on Stewart's show), what makes this special is the addition of Conan of Late Night with Conan O'Brien. The three wound up having a mock feud over then-Presidential candidate Mike Huckabee, culminating in a Melee a Trois on Conan's show.
 * Colbert and Jimmy Fallon each had the other crossover in the promotion of a fundraiser competition. It culminated in a performance of Rebecca Black's "Friday" that contained nearly the legal limit of awesome.
 * Casualty and its spin-off Holby City have had four crossovers titled Casualty@Holby City (although they were given a special title, they ran as two-parters in the usual slots): two Christmas Episodes, one Halloween Episode, and a Very Special Episode Organ Donation Awareness Week episode. We have yet to see Casualty@Holby City With Holby Blue, but doubtless it's coming...
 * And it has: although it wasn't called Holby Blue@Holby City, the first episode of Season Two of Holby Blue was a direct continuation of that week's Holby City, both of which featured the medics and the cops.
 * The various Star Trek series saw many of these, beginning with The Next Generation, although events in one series rarely affected the others. The crossovers became more frequent in later years.
 * CSI has crossed over with two other shows within the CSI Verse, CSI: Miami and Without a Trace. Miami, meanwhile, has crossed over twice with CSI New York, and New York once did so with Cold Case, and the three core CSI programs have recently crossed over with a three-part story featuring Ray Langston. All five are produced by Jerry Bruckheimer and air on CBS, so bringing them together is less complicated than most.
 * All My Children, One Life to Live, and the defunct Loving were all created by Agnes Nixon and share a universe. Numerous crossovers have occurred over the years. In 2004, AMC and OLTL launched an ambitious crossover storyline in which a baby from All My Children was kidnapped to One Life to Live. The story lasted several months.
 * ABC also had a story in 1999 (essentially a ratings stunt) in which Linda Dano's One Life to Live character traveled to All My Children, Port Charles, and General Hospital searching for her long-lost daughter, who turned out to be a fellow soap-hopping character, Skye Chandler.
 * Two Pints of Lager and a Packet of Crisps is crossed over with fellow BBC Three shows Grown Ups (which also stars Sheridan Smith, who plays Janet in Two Pints) and Coming of Age for the show's Comic Relief special.
 * In a desperate attempt to prop up the ratings of its woebegone late '70s sitcom Hello Larry, NBC had the cast make numerous crossover visits to the popular Diff'rent Strokes.
 * Happy Days and Laverne and Shirley had characters appearing on each other's shows fairly regularly, and there was one true crossover storyline that spanned episodes of both series -- which was only reasonable, since Laverne and Shirley was a Spin-Off of Happy Days. And just to add a little weirdness to the mix, Mork and Mindy was also a Happy Days spinoff...
 * The Golden Girls, Empty Nest, and Nurses were all made by the same production company and all shared the same Miami setting, so there was predictably a fair amount of crossover among them.
 * Magnum, P.I. had crossovers with Murder, She Wrote and Simon and Simon.
 * Las Vegas and Crossing Jordan crossed over with one another on a regular basis.
 * Everybody Loves Raymond, The King of Queens, and The Nanny also exist in the same universe and have been known to cross over.
 * A rather famous "Night of Elizabeth Taylor" also tied The Nanny to the other shows on the network that night, Can't Hurry Love, Murphy Brown, and High Society.
 * Talk about puking on a pile of shit.
 * Two David E. Kelley series, Ally McBeal and The Practice, had a 1998 crossover storyline that encompassed episodes of both shows.
 * The continuity was thrown for a loop when Peter MacNicol played a one-off character on Boston Legal, a spin-off of The Practice, after having played a lead character on Ally McBeal.
 * Friends crossed over with The Single Guy, as well as with Mad About You. (A minor character on the latter show, Ursula the waitress, is actually the sort-of-evil twin sister of Friends' Phoebe.)
 * Also, there was a a time when Chandler knew Annie Spadaro, of Caroline in The City, that is too a Cameo of the former in this series.
 * Characters from The Love Boat guest-starred in episodes of Charlie's Angels and Martin.
 * Seinfeld and Mad About You crossed over at least once, which generated Fridge Logic when in an episode of the former two characters were seen watching the latter.
 * There hasn't been a direct crossover between The Shield and Sons of Anarchy, but a gang called the One-Niners, based in Los Angeles, has appeared in both shows, indicating that the shows share a continuity. Considering that one of Sons of Anarchy's producers was a writer on The Shield, it's not surprising.
 * All of the series in the DCLAU tend to have numerous crossovers with eachother. In 2006, That's So Raven, Hannah Montana, and The Suite Life of Zack and Cody have crossed over (in a special called That's So Suite Life Of Hannah Montana), and, in 2009, there was an another one with Hannah Montana, The Suite Life On Deck, and Wizards of Waverly Place (Wizards On Deck With Hannah Montana). Interestingly enough for shows with little enough continuity, the characters from The Suite Life and Hannah Montana recalled the previous crossover.
 * Also, Zack and Cody got to meet Iron Weasel when they snuck aboard the S.S. Tipton in an episode of I'm in The Band.
 * Prior to that there was a rather famous one in 1996 with Sabrina the Teenage Witch, Boy Meets World, Teen Angel, and You Wish where the wise cracking wizard turned cat Salem uses a temporal ball and starts some shear fun.
 * There was an episode of Power Rangers that crossed over with Teenage Mutant Ninja Turtles.
 * In the realm of examples that don't inflict pain in the viewing audience, from Lost Galaxy on, there's a recurring tradition of the seasonal team-up, a two-parter where the previous season's cast meets up with the current Rangers to deal with a threat that requires their combined powers.
 * There is also a razor-thin line connecting Power Rangers with Doctor Who. In an episode of Lost Galaxy, a set of coordinates to a meteor field are given, which are the same coordinates as Gallifrey (which probably would be nothing but a meteor field now, considering the Time War.) Doctor Who was off the air at the time, but to reciprocate, a Big Finish Audio book referenced a Zordon Nebula. Maybe somebody who works on either show could strengthen this for us.
 * Ultraman Mebius has crossovers with older/the original Ultra Series throughout its run. Be it from classic monsters to the older heroes showing up for a team up. The Movie, Ultraman Mebius and the Ultra Brothers takes this to 11.
 * Jack Webb's triumvirate of Adam-12, Dragnet, and Emergency featured several crossovers between the three.
 * The Polish Crime and Punishment Series W-11 and Detektywi made at least one such crossover: an episode on Detektywi was the first part, depicting the eponymous detectives investigating a case; then, for the second part, the policemen of W-11 took over.
 * Eureka and Warehouse 13 have a pair of crossover episodes, where Fargo upgrades the Warehouse's computer system and Claudia visits Eureka. Fargo even managed to MacGyver a lightsaber with a laser pointer and an artifact.
 * Sy Fy even used these episodes for a canon crossover ship. Claudia/Fargo.
 * This also now includes Alphas.
 * For one Comic Relief event, British Panel Games Have I Got News for You, Never Mind the Buzzcocks, and They Think Its All Over crossed over. The product was, of course, titled Have I Got Buzzcocks All Over.
 * An earlier Comic Relief had Have I Got A Question of Sport Fot You.
 * In The Man From UNCLE's heyday an episode of Please Dont Eat The Daisies had the children believing their father was a spy after seeing him pass a matchbook to U.N.C.L.E. agent Illya Kuryakin in an innocuous (or was it?) encounter. Hilarity ensues until, in the show's concluding scene, someone is brought in to convince the children that they've jumped to a foolish conclusion. It is, of course, Robert Vaughn whom the kids recognize instantly as Napoleon Solo.
 * There was also an episode featuring The Girl From UNCLE, April Dancer.
 * Hercules: The Legendary Journeys and Xena: Warrior Princess. You know Xena has Jumped the Shark when.
 * Commercial example involving Monks Adrian Monk and The Dead Zones Johnny Smith. Three versions of the promos have them avoiding touching other people/things (at a detectives association meeting) and exhibiting their Blessed with Suck/Cursed with Awesome (Monk's OCD on corn counting and Smith's psychic ability with a punch bowl). The USA Network aired both of their shows at the same time.
 * Word of God says that if Psych hadn't made Leverage a fictional show within Psychs universe, Leverages Eliot would have had an uncle named Henry.
 * The WB once had a night dedicated to crossing over their shows. A character from one show would pop up in another, mostly for laughs and night contribute much to episode's plot. It was parodied in that night's episode of Unhappily Ever After, wherein Jackee Harry and hinted that The Wayans Bros were "waiting in the car".
 * Breakout Kings : In the third episode of the first season, Prison Break's T-Bag....breaks out from a prison. An unprecedented case of a character crossing over to a show on a completely different network but not so surprising considering both show have the same producers (and an obvious common theme).
 * Kate Monday (or was it Pat Tuesday?) and George Frankly from Square One TV's Mathnet appeared on Where in the World Is Carmen Sandiego?.
 * Captain Kangaroo and Mister Rogers made appearances on each other's shows.
 * While at the beach, Logan and Carlos of Big Time Rush meet up with Patchy the Pirate of SpongeBob SquarePants
 * Mikey and Paul Jr. from American Chopper head down to Baton Rouge to hang out with the guys from Sons of Guns.
 * In the ninth season finale of Two and A Half Men (the one after Charlie got McLeaned), among the people looking to buy Charlie's house are Dharma and Greg.
 * The Six Million Dollar Man and The Bionic Woman had regular crossovers, with two-part and three-part stories involving both shows (most notably "Kill Oscar," which had parts 1 and 3 on Jaime Sommers's show with the middle part on Steve Austin's). Curiously, although both series aired on ITV in Britain, the crossover episodes weren't treated as such: all three parts of "Kill Oscar" and both parts of "Welcome Home, Jaime" were aired as episodes of The Bionic Woman, while both parts of "The Return Of Bigfoot" were shown via The Six Million Dollar Man.
 * Petticoat Junction and Green Acres were set in the same loose universe, and characters from each gravitated to the other fairly regularly. And a 1968 Thanksgiving episode of The Beverly Hillbillies had that show's characters traveling to Hooterville, leading to a three-way crossover between the shows. All three rural comedies aired on CBS and were developed by Paul Henning.
 * USA network had a set of commercials crossing over their various shows; Adrian Monk and Johnny Smith are each thankful the other doesn't want to shake hands; Johnny and Shawn Spencer debate who has the worse past; until they notice Monk counting his food; etc.

Mythology

 * Older Than Feudalism example: the classical tale of Jason and the Argonauts brought together many mythological heroes in a sort of Justice League Ancient Greece. Of course, "hero" in Classical Mythology was an awfully vague term (as Larry Gonick put it, heroes aren't necessarily heroic, just excessive), and since the mission was to steal an item (the Golden Fleece) "Justice League" isn't a terribly good name for them....
 * Maid Marian, the shepherdess, featured in many pastoral plays popular at May festivities. Then there was a fad for Robin Hood plays. Then someone put the two characters together. It worked so well that Maid Marian is best known as a Robin Hood character, even though her independent existence predates him.

Radio

 * An arguable one, similar to the Comic Relief Panel Game crossovers above, the New Year 2010 episode of The BBC Radio 4 comedy panel game The Unbelievable Truth featured Stephen Fry, Alan Davies, regular QI panelist Rob Brydon and QI producer John Lloyd. The format of the game was unchanged, but the "obvious answer" klaxon was added. Much to the satisfaction of the other players (and host David Mitchell), Stephen set it off twice.
 * The Archers had a crossover with Gardener's Question Time for Easter 2011, where the real-life panel, chaired by the real-life Eric Robson answered questions from the population of Ambridge.

Tabletop Games

 * "Reality Storm: When Worlds Collide" crossed over Hero Games' Champions and Guardians Of Order's Silver Age Sentinels, co-produced by both companies. The adventure itself was dual-statted, but the book also included official conversion notes between the Tri-Stat d10 system and 5th edition Hero System.
 * Dungeons and Dragons did this with the second edition meta-settings Planescape and Spelljammer connect individual settings' worlds into one Multiverse. So the characters of Forgotten Realms, Greyhawk and Dragonlance can take a ride and visit each other. Planar travel reaches isolated places such as Dark Sun and Ravenloft. The latter hardly is a desirable destination, so it has The Mists that can appear in normal worlds and abduct the PCs to the Demiplane of Dread.
 * The Spelljammer novels of The Cloakmaster Cycle demonstrated such a campaign, with most of the first book taking place on Krynn, only gradually introducing the concept of spelljamming to the hero. Later he visited two of less known locations on Toril. At least Forgotten Realms actively supported the meeting with other worlds by dropping mentions in its own sources.

Theatre

 * A more recent example (1829) is the theatrical play Don Juan und Faust by Christian Dietrich Grabbe, a German dramatist. It is a crossover of the myth of Faust and of the tale of Don Juan.

Video Games

 * Konami has a great story with this.
 * Konami's Wai Wai World series, which was the first cross-over in video game history.
 * Konami's Parodius series.
 * Konami Krazy Racers.
 * Dream Mix TV World Fighters, is a crossover between Konami, Hudson and Takara.
 * The Vic Viper appears as a Transforming Mecha in Zone of the Enders 2.
 * Some characters in Pop N Music come from other Konami franchises, as a result of having songs or remixes of songs from games they appeared in being put in the game:
 * Vic Viper, in his NES Gradius form ("Gradius -Full Speed-", "A Shooting Star")
 * Simon Belmont ("Akumajo Dracula Medley Hybrid".)
 * Goemon ("Ganbare Goemon Medley")
 * Emi and Afro, exactly as they appeared in DDR ("Flowers" and "Afronova Primeval," respectively, as well as "DDR Megamix" for Afro)
 * Tran (various IIDX songs)
 * SigSig, exactly as she appeared in IIDX ("SigSig")
 * Dingo and Ken ("Beyond the Bounds")
 * Adventure Quest Worlds has a Crossover event with Ctrl Alt Del.
 * AdventureQuest Worlds} itself is one huge mix of the three main AE games, Adventure Quest, Dragon Fable, and Mechquest. Heroes and villains from all three have been featured in the game, although arguably the most evil villain of all three gets killed off right in the beginning of the game.
 * Before that, DragonFable and AdventureQuest had a crossover mission.
 * Also, AdventureQuest and Warp Force are in the same period of time, but it began with a crossover from AdventureQuest.
 * The King of Fighters -- Featuring characters from Fatal Fury, Art of Fighting, Psycho Soldier, Ikari Warriors, and one or few from other SNK titles, as well as some exclusives like Kyo Kusanagi and his partners and rivals.
 * One game in its 3D spinoff Maximum Impact (Mainly MI:2) had Hanzo Hattori from Samurai Shodown and Fio Germi from Metal Slug as unlockable characters.
 * In addition to some already present in the KOF series, Neo Geo Battle Coliseum also added characters from other SNK titles that the KOF series didn't add, as well as some ADK titles (due to SNK Playmore's purchase of ADK after it filed for bankruptcy).
 * Kingdom Hearts -- Where Disney and Final Fantasy meet.
 * The Rival Schools games have had a few crossovers with Capcom's more popular fighting game series Street Fighter; the most obvious was Sakura Kasugano, Ryu's Hero Worshipper, being a playable character in the original game. There is also a character known as Ran Hibiki, whose name is almost exactly the same as that of Dan Hibiki. Just what her relationship to him is undetermined, assuming there is a relationship at all and it is not simply an Homage.
 * Ran's classmate, the unnamed Chairperson, also happens to study Dan's self-invented martial art, Saikyo-ryu, with predictable results.
 * Hinata also claims to be a student of "Masters-ryu Karate" in Nekketsu Seishun Nikki 2, which explains why some of her special moves have the same flame effect as Ken's Dragon Punch in Super Street Fighter II.
 * Also in Street Fighter, Cody, Guy, Rolento, and Sodom from Final Fight appear as playable characters in the Alpha series. The crossover between the two also carried over into the cartoon, which featured a retelling of Final Fight's storyline while adding Ryu and Ken.
 * Super Robot Wars. That is all.
 * ...Until the same company also gave us Another Century's Episode.
 * Despite being an internally consistent multiverse, the Nasuverse only has crossovers in incredibly round-about ways. Part of the problem is, well, Word of God stated that some of the characters simply cannot exist in the same universe for Canon reasons.
 * Zelretch was seen in the Prologue to Tsukihime with a young Arcueid. He also appears for one scene in the "Heaven's Feel" route to Fate/stay night, literally walking in from Another Dimension to meet Rin.
 * Aoko Aozaki spends time in the beginning of Tsukihime with a young Shiki Tohno. Her sister, Tohko Aozaki, is the Older and Wiser of (arguably) Tsukihime-prototype, Kara no Kyoukai. Despite each of them knowing a different Shiki (each possessing the Mystic Eyes) it's explicitly stated that Tohko and Arcueid cannot exist in the same universe (and most likely, Shiki Ryougi and Shiki Tohno can't, either).
 * Shiki T.'s eyes are a genetic thing from the Nanaya. Shiki R.'s are really Mystic Eyes. I don't know what's up with Tohko and Arcueid.
 * It's more along the lines of "Tohko's goal contradicts with the existence of True Ancestor Arcueid", though Nasu hasn't really made it clear on why. And Mystic Eyes are just... eyes with special powers. Many, many characters have them, just that there can't be more than one possessor of the Eyes of Death Perception.
 * Shiki's Eyes of Death Perception is different from the genetic eyes of the Nanaya. Since both Shikis are from closely associated families of assassin though, it might be related to genetic. However Ryougi's family does not possess special eyes. The reason why only one can exist might be due to the nature of those eyes. Given that they are potentially insanely powerful and you can't just have people running around with such things, it might be a case of The Chosen One.
 * It's just chance. The Eyes of Death Perception have a rare chance of showing up in their respective families. It's not that they can't exist in the same universe, it's that the chances are very, very low.
 * The adventure game Sherlock Holmes: Nemesis features Holmes matching wits with Gentleman Thief Arsene Lupin.
 * The are stories and videogames which also cross him over with the Cthulhu Mythos.
 * The Sega Superstars series subverts this at first, as it is essentially a collection of games based on Sega properties, but the second game, Superstars Tennis has Sega characters all play tennis together, playing the trope straight. Then there was a thrid racing game, which was a lot better than expected.
 * Before that we had, Fighters Megamix, which included most of Sega's 3D characters on a fighting game.
 * While it makes sense since they are a part of the same cosmology, most of Bioware and Obsidian's D&D-based games have subtle crossovers. Most notably, Baldur's Gate II features artifacts from some of the characters of Planescape: Torment, and one of said characters is also mentioned by an NPC in Neverwinter Nights 2.
 * Not surprising. The collusion started with Neverwinter Nights, but reached its height with Knights of the Old Republic. The first game is "Baldur's Gate with lightsabers." The second is "Planescape: Torment with lightsabers"
 * Perhaps less crossover than Cameo but the fates of the character from the TV show of Dungeons and Dragons is on a wall in Baldur's Gate II.
 * Ryu Hayabusa in Dead or Alive. Also done in reverse, with Ayane appearing as a supporting character in the new Ninja Gaiden games.
 * Battletoads and Double Dragon: The Ultimate Team was one of the earliest intercompany video game crossovers, which was made possible by the fact that both series were published in America and Europe by Tradewest. The game itself is really a Battletoads game with Double Dragon characters as guest stars, (some which were misnamed).
 * Super Smash Bros.
 * Mario and Sonic At The Olympic Games
 * Knights is featured in both of the Sonic Adventure games, and while he bears no real significance to the plot, he is a part of a level in each, moreso in Sonic Adventure.
 * Jewelry Master Twinkle, an already-obscure Puzzle Game, is actually a crossover between the Tetris TGM-like game Jewelry Master and the even-more-obscure mahjong game Taikyoku Mahjong Net-De-Ron.
 * Unreal Championship 2 The Liandri Conflict featured Raiden from Mortal Kombat and Shao Kahn as an optional announcer. It helps that it was the first Unreal game being published by Midway Games.
 * Capcom has something of a history with these, with Marvel vs. Capcom 3 being the latest installment.
 * Mortal Kombat vs. DC Universe
 * Aliens vs. Predator. Unlike the films, this series is actually really good. The Atari Jaguar game and the two PC games are considered classics in the FPS genre.
 * The Soul Series is famous for this. Guest characters have included Link, Spawn, Heihachi, Lloyd Irving, Kratos, Yoda, and Darth Vader.
 * Cross Edge is this in and out.
 * Astro Boy Omega Factor is a Crossover action game of Astro Boy and other Osamu Tezuka's works.
 * Professor Layton x Ace Attorney = Happiness^2
 * A (currently) tentative title game with a three-way crossover with Jak and Daxter, Ratchet and Clank, and Sly Cooper. Some fans were thrilled, others...Not so much.
 * Dissidia Final Fantasy. It's basically the main heroes and villains of Final Fantasies I-XII beating the ever-loving crap out of each other!
 * The retail version of the Do Don Pachi DaiFukkatsu Black Label 360 port features an arrange mode that crosses DoDonPachi over with Ketsui, with scoring elements, player ship, and True Final Boss taken right out of Ketsui.
 * Famicom Jump 1 and 2.
 * Jump Super Stars
 * Putt-Putt and Fatty Bear's Activity Pack.
 * Koumajou Densetsu, Touhou meets Castlevania.

Web Original

 * Lonelygirl15 crossed over with OpAphid v. Tachyon in season one and Kate Modern in season three.
 * The feud between The Angry Video Game Nerd and The Nostalgia Critic?
 * The subsequent massive crossover with a bunch of gamers/reviewers?
 * Not only does Doctor Insano end up in Linkara's videos often, he first appeared in one.
 * Bandit Keith also appears in a few of Spoony's videos, portrayed by Little Kuriboh
 * Super Mario Bros Z crosses the universe of Super Mario Bros. with that of Sonic the Hedgehog. Not to mention its one big love letter to Dragonball.
 * How It Should Have Ended gives their take on how the Terminator series should end. With an unexpected crossover.
 * Lemon Demon's "Ultimate Showdown of Ultimate Destiny".
 * Roosterteeth and Mega64 Finally teaming up this week (as of Monday, November 29th, 2010). Though they have run into each other before.
 * I'm a Marvel And I'm a DC.
 * The Anti Cliche and Mary Sue Elimination Society actually uses Crossovers as a gadget. It can temporarily fuse two fandoms together, but it isn't used all that often.
 * A few of the Abridged Series' have crossovers. Examples include Ghost Nappa appearing in a Yu-Gi-Oh!: The Abridged Series video, and Yami Yugi appearing at the end of the Dragonball Kai: The Abridged Series video.

Web Comics

 * The main character in the webcomic Here Wolf debuted in the comic book Horndog, and there are occasionally crossovers from time to time.
 * Irregular Webcomic does this from time to time, sometimes in ways you might not imagine.
 * Least I Could Do recently had a huge one with characters from just about every video game webcomic (and a few others), all fighting each other to the death. Rayne Combatants included Dinosaur Comics, Eight Bit Theater, Looking for Group, Ctrl+Alt+Del, Penny Arcade, xkcd, Cyanide and Happiness, and many, many others. Also, John dressed as the Green Lantern and Rayne's car became a Transformer. And he got it on with some superhero babes.
 * Sadly, the long-awaited battle between Black Mage and Richard the Warlock never happened since they were apparently on the same side, but 's not to say it never will. You never know with Sohmer and Lar.
 * Props if you noticed
 * Something Positive (and all of Randy's other strips by proxy except for Super Stupor), Queen of Wands (and Punch an Pie by proxy), Penny and Aggie, Questionable Content, Scandal Sheet!, and Girls with Slingshots are all part of the same webcomic universe. Using S* P as a node, this is the beginning of the Grand Unified Webcomic Universe.
 * And now, Shortpacked! and the rest of the Walkyverse by proxy. (Although a previous two-sided crossover between Shortpacked! and Questionable Content had already unofficially put it into the S*P Multiverse.)
 * It's been established in The Inexplicable Adventures of Bob that Molly likes to write crack crossover fanfics, like The Brothers Karamazov meet Harold and The Purple Crayon (Harold uses his purple crayon to break Dmitri out of a Siberian prison). One of her fics got illustrated to appear on Dr. McNinja's website, thusly.
 * The strip itself has had brief cameos by characters from Melonpool, Zortic, Everyday Heroes, and Zeera the Space Pirate.
 * One of the earliest webcomic crossovers was a week of strips in 1998 between User Friendly and Sluggy Freelance where Torg temps at the ISP while Riff ends up battling the Coffee Monster.
 * Emergency Exit is quite famous for its very extensive crossover with Parallel Dementia, notably having a drastic effect on the plots of both series. It was also very, very long. Emergency Exit also had a crossover with Beyond Reality before that (Though that crossover didn't really effect the plot at all) and later had a second crossover with Parallel Dementia that consisted largely of Fall and Eddie having a conversation, with several big revelations made.
 * Jix has at least two crossovers from the creator's other series Dragon City. Both of which involve stories told about encounters with dragons, both of which were also mentioned in Dragon City, but without the Jix characters. Both times it involved main characters.
 * Also, in Dragon City, there were four major crossovers from Jix. One was when an Ambis (the alien species from Jix) tried to take over the dragons' world, another instance when one of the villains tried to kill Erin because he thought the previous crossover was still disguised as her, one was when Jonas got kidnapped by one of Jix's minor villains, and the last was when Lauren and Jix saved Erin from her editor.
 * In summer 2010, the world was introduced to an Axe Cop and Dr. McNinja crossover. And it was awesome.
 * Fans had a crossover with Penny and Aggie recently, or at least the version of Penny and Aggie that exists in the Fans-verse.
 * Nerf Now explains the reasoning behind this.

Western Animation
"[Homer and Bart accidentally pick up Bender of Futurama while driving through a portal tunnel]
 * Hanna-Barbera did this a lot. Scooby Doo was usually the host show. Other Hanna-Barbera examples are Laff-A-Lympics and Yogi's Gang.
 * The Jetsons Meet the Flintstones was one of the few movies they did of this.
 * Perhaps the most notorious use of this was Scooby Doo in Arabian Nights.
 * There was a mutli-episode arc of Space Ghost where the title character went up against the Council of Doom, each of whom managed to defeat Space Ghost and banish him to some other realm/time/planet, just so the other Hanna-Barbera heroes of that setting could help save him. This allowed Space Ghost to team up with the Herculoids, Mightor, Shazzan and even Super Moby Dick.
 * The 1981-82 Hanna-Barbera show Space Stars was also built around this trope. Each hourlong episode would have a short eight- to 10-minute segment starring one of the show's hero teams: Space Ghost, The Herculoids, the Teen Force and even Astro from The Jetsons. The last five to 10 minutes of the show would then be a "Space Stars Finale" team-up between two or more of the groups against a common foe.
 * Johnny Bravo, "Bravo-Dooby-Doo": Johnny hitches a ride with the cast of Scooby Doo to his great-aunt's mansion. At the end, Speed Buggy shows up.
 * A later episode has a past incarnation of Johnny meeting Fred Flintstone.
 * The later seasons had a lot of episodes revolving around him meeting other cartoon characters. Blue Falcon, Huckleberry Finn, Weird Al...
 * Yogi Bear and Boo Boo once turned up in a Flintstones episode. Apparently, those bears are really, really old.
 * The Venture Brothers started off as a parody of Jonny Quest. Eventually, characters from Jonny Quest started showing up on the show... or at least demented versions of them.
 * Matt Groening's shows The Simpsons and Futurama tend to crossover for brief instances, including in the futuristic Simpsons episode "Future Drama".

Bender: Oh boy! You guys are gonna be my best friends, right?

Homer: You wish, loser!

[Throws Bender out the car.]"

"TV Announcer: Coming up next, The Flintstones Meet the Jetsons.
 * Then, of course, there was the time The Simpsons crossed over with The Critic.
 * See The Simpsons episode "A Star Is Burns" for that one.
 * The 1987, 2003, and original comic book versions of the Teenage Mutant Ninja Turtles, in Turtles Forever.
 * Also, Miyamoto Usagi appeared in both TMNT cartoon series.
 * Spider-Man: The Animated Series had a crossover with the 90's X-Men animated series. It was considered a big deal because it was a completely different animation studio involving the then current roster from X-Men in a show that was not their own. Even more impressive was the effort put in to keeping all the same cast (save for one, Gambit, presumably for contractual or scheduling reasons) for the sake of continuity. Even more fun, the crossover remains in continuity for Spider-Man, as Storm returns during the series' adaptation of the Secret Wars crossover event.
 * The Marvel cartoons from around this era were frequently cameo-ing in each other's series, as well (though it's hard to know if they were literally the same characters as the other cartoons; they all take place in a Marvel Universe, where a Spider-Man, Human Torch, etc. would likely exist somewhere.) You never know who'll be briefly shown watching from a rooftop, or looking up at the Pillar of Light in the distance when something really big goes down. Also, Iron Man, Fantastic Four, Incredible Hulk, etc. guest starred in each other's shows often enough to make the 90s Marvel cartoons a Diniverse of sorts - you can connect the dots through all of them.
 * It was a bit more subtle than most of these examples, but G.I. Joe crossed over with Transformers Generation 1 in the episode "Only Human". A masked character named "Old Snake" (voiced by Chris Latta) helps the Big Bad of the week with a machine to put Transformers' minds into synthetic human bodies. After the plan inevitably fails, Old Snake escapes and idly muses that "they just don't make terrorists like they used to", then raises his arms and yells "COBRAAAAAA!", ending in a coughing fit.
 * This has a hard time fitting in with the events of G.I. Joe: The Movie
 * And from the same season, recurring character Marissa Faireborne was obviously the daughter of Flint (Dashiell R. Faireborne) and Lady Jaye. The DVD commentary for Transformers: The Movie finally admitted this officially.
 * One "cameo" was made when a hologram version of an aged Flint was used in order to trick Marissa in one episode.
 * Old Snake, you say?
 * The all-too-trippy anti-drug PSA Cartoon All-Stars to the Rescue?
 * The Simpsons crossed over with The Critic in the episode "A Star Is Burns". This was even lampshaded in the episode itself, which additionally, they did some Leaning on the Fourth Wall in favor of The Critic's Show Within a Show of which Jay is the host:

Bart: Uh oh. I smell another cheap cartoon crossover.

Homer: Bart Simpson, meet Jay Sherman, the critic.

Jay: Hello.

Bart: Hey man, I really love your show. I think all kids should watch it! (turns away) Ew, I suddenly feel so dirty."

"Lisa: Goodbye, Mr. Sherman. If I ever play Carnegie Hall, I'll give you a call.
 * Later...

Jay: And if you ever want to visit my show ?

Bart: Nah, we're not going to be doing that."

"Thundarr: Can no leader go untainted?!"
 * The Fairly Odd Parents and Jimmy Neutron Boy Genius crossed over three times. This was an interesting exercise because one show is Thick Line Animation while the other is computer-generated, requiring the characters to be re-animated in the appropriate style while in the "other" show's universe.
 * Also notable for the "magic vs. science" theme in all three specials.
 * Eventually, this led to the Nicktoons Unite! video game series, which had those three specials as their basis (and made some continuity nods towards them).
 * Cartoon Network has done many promo spots for itself using odd and unusual crossovers such as Shaggy from Scooby Doo and Droopy Dog discussing what they call The Pound Puppies in France while driving a (live-action) convertible in (live-action) L.A., à la Pulp Fiction.
 * Another showed a group of sidekicks discussing their position at Hadji's, while Chicken protested that he was a co-star, not a sidekick. "My name is in the title, for cryin' out loud!"
 * There's one line of promos they did called "Perfect as It Is", which show that certain cartoon character cross pairings just don't work out right. Take a look at "Tom and Speedy", "Road Runner & Dexter", and "Batman and Inch High Private Eye" to see what they mean by that.
 * Cartoon Network liked the concept of having their characters in one workplace. It's even shown on the Friday Nights intro.
 * "Tom and Speedy" could work. After all, Speedy Gonzales' nemesis is Sylvester, which is just Tom with a lispy voice.
 * The Superfriends and The Powerpuff Girls here.
 * Yet another had Fred Flintstone, Chicken, and Thundarr the Barbarian returning from lunch and futilely searching for a parking space. Thundarr is incensed that Papa Smurf has a reserved space:

""WORST. CAMEO. EVER!""
 * You can also see them still going to work together in the Friday nights intro.
 * A Ben 10 and Generator Rex crossover titled "Heroes United" is aired on Cartoon Network during Thanksgiving 2011, as a "special, extended" episode of Generator Rex. A clip was shown at Comic-Con 2011, in which Ben is seen entering Rex's world through a portal, and is at first mistaken for an EVO.
 * In a way, one could look at this crossover as a 'buddy cop' event, in which the white guy(Ben) and the Hispanic(Rex) team up to fight whatever evil it is they'll be fighting. The fact that both characters are working for government-esque agencies definitely makes the team-up more buddy-cop.
 * Rugrats and The Wild Thornberrys had a feature-film crossover.
 * Harvey Birdman, Attorney at Law is nearly built around this trope, with most episodes involving Harvey's law firm taking the case of some 1960s or 1970s cartoon character.
 * Lilo and Stitch: The Series has crossed over with several other Disney Channel original cartoons, including The Proud Family, Kim Possible, American Dragon Jake Long, and Recess.
 * A crossover of Darkwing Duck with Chip 'n Dale Rescue Rangers was planned but was never produced beyond a few voices. A quick clip of these voices can be heard in the Darkwing episode "Twitching Channels".
 * Darkwing Duck did have DuckTales' Gizmoduck as a reoccurring character though.
 * And in one episode, the entire recurring villain cast from DuckTales showed up in one scene.
 * Also in the same vein, Hercules crossed over with Aladdin.
 * Pepper Ann is the only animated Disney television show to ever be covered in the show House of Mouse.
 * Freakazoid, Animaniacs, and Pinky and The Brain did a crossover in this clip.
 * Animaniacs sometimes did crossovers with other cartoons on its show, including one incident where Batman rescued them from an angry fairy (as Yakko's Shakespeare recital had foretold). Coincidentally, Batman the Animated Series aired right afterward.
 * Tiny Toon Adventures was also in this same mold (considering both shows were from the same producer, Steven Spielberg). In fact, some Animaniacs characters made quick cameos in the two prime-time Tiny Toons specials in 1994.
 * Additionally, the Looney Tunes characters made several guest cameos on Animaniacs and Histeria!
 * Superman and Batman (in their Timm/Dini incarnations) crossed over multiple times, starting with the World's Finest three-parter. Eventually, this led to the DCAU, which had this trope as its foundation.
 * South Park did a Cartoon Crossover where Mr. Garrison sought therapy from Dr. Katz.
 * Interestingly, Mad did the very same crossover earlier.
 * Also the "Imaginationland" trilogy could also be considered something of a massive crossover as well.
 * "The Grim Adventures of Billy and Mandy of the KND" was really more of a Kids Next Door episode with Billy and Mandy characters, but many plot points rely on the main characters of the latter (such as the Applied Phlebotinum). There were also many a Shout-Out to other Cartoon Network original properties, including a brief Art Shift to the world of Ed, Edd n Eddy.
 * Fred Flintstone made a guest appearance in an episode of The Grim Adventures of Billy and Mandy strangely enough, though no one ever referred to him by name.
 * He was referred to as "Jake Steele!" and spoke only in "Yabba dabbas"..
 * In the same light, Yogi Bear and Boo Boo showed up in GABM as well.
 * Inspector Gadget once paid a visit to the live-action half of The Super Mario Bros Super Show, played by a svelte young Maurice LaMarche.
 * An arc of Street Sharks dealt with them teaming up with the Extreme Dinosaurs.
 * Ace Ventura and The Mask once teamed up as cartoons. Particularly bizarre since both characters were played by Jim Carrey in the live-action movies.
 * Back in 1972, The ABC Saturday Superstar Movie anthology series featured Looney Tunes characters in a Filmation production, Daffy Duck and Porky Pig Meet The Groovie Goolies -- an atypical crossover of characters from different animation studios. (Consider that veteran WB animators, such as Ted Bonnicksen, Virgil Ross and Norm McCabe were all working at Filmation at the time.)
 * Earlier in the series, King Features had produced a film, The Man Who Hated Laughter, which featured a crossover of characters such as Blondie, Beetle Bailey, Flash Gordon, Hi and Lois, The Katzenjammer Kids, Mandrake the Magician, The Phantom, Popeye, Prince Valiant and Snuffy Smith from the syndicate's various comic strips.
 * The main characters of Defenders of the Earth were all from previous series, making the entire series a Crossover.
 * Flash Gordon! The Phantom! Mandrake the Magician! These three heroes united by a quest for justice and the coincidence they're all owned by King Features Syndicate now defend the Earth as... the Defenders of the Earth!
 * Alvin and The Chipmunks once had a crossover with... Alvin and The Chipmunks. Or rather, the 1980s cartoon chipmunks got to meet their 1960s counterparts in a manner involving time travel, similar to Turtles Forever.
 * And one of the show's couch gags featured The Simpsons meeting their Tracy Ullman Show counterparts sitting on the sofa.
 * Seth MacFarlane's Family Guy, American Dad, and The Cleveland Show.
 * The Batman vs Pokémon promos on Kids WB. Part One and two.
 * Several other promos running on Kids WB featured similar interactive crossovers between the block's various shows.
 * Building on the Scooby example above, the 14th episode of Scooby Doo Mystery Inc had Scooby team up with Captain Caveman and the Teen Angels, Jabberjaw, The Funky Phantom, & Speed Buggy in an Affectionate Parody of old Hanna-Barbera cartoons.
 * It seems that one of the animators that exists in The Cleveland Show created a animated version of a Na'vi from Avatar.
 * Comic Book Guy from The Simpsons cameos in one episode.


 * A brief subtle cross over in The Whiteboard, a quantum coffee maker dinosaur pays a visit here.
 * An episode of The Zeta Project crossed over with Batman Beyond, justified in this case as the former was a spinoff of the latter. (Due to Bob Kane's contractual billing being what it is, this is also the only episode where the opening titles omit the "Created by Robert Goodman" credit - the end credits specify Kane's being behind Batman, with Goodman being behind the characters for the spinoff.)