Cameo Prop

Many writers are fanboys, just like us. That said, it is not unreasonable for them to want to throw in some reference to one of their favorite works. Sometimes, however, said element is a bit more blatant, and a lot more physical. This is the idea behind a Cameo Prop. It originally has nothing to do with the work of fiction or the series it is appearing in, but, usually, for sake of novelty, someone throws it in.

Let's say an episode of a show involves Bob going to an auction and winning the Ruby Slippers worn by Judy Garland when filming The Wizard of Oz. The rest of the episode might involve Bob wearing them (much to everyone's amusement), and even reenacting scenes from the movie. Almost always, he'll end up losing them in the end.

A cameo prop is specific: Having a Red Stapler or (especially) Aluminum Christmas Trees does not make it a cameo prop. Here's some examples to help make the difference clear.

Example A: A DMC DeLorean appearing in a Die Hard movie. This model of car is famous for being that of the car from Back to The Future, however this is an ordinary DeLorean. Not a Cameo Prop.
Example B: The DeLorean from Back to The Future appearing in a Die Hard movie, complete with all the gear on the back, flux capacitor between seats, time gauge, everything. A Cameo Prop.

Examples of Cameo Prop include:

Film

The first Inspector Gadget movie, when the character receives his trademark outfit. In the cartoon, the outfit was a Shout-Out at best. In the movie, however, he directly identifies it as the Gadget Suit from Get Smart.
Evil Dead II famously has the Freddy Krueger glove hanging from a wall.
The Hitchhiker's Guide to the Galaxy: The film version has a scene where one of the background characters is wearing the Marvin costume from the television version.
RoboCop's trademark Auto 9 pistol appears in Sin City.
Looney Tunes: Back in Action features several famous movie and TV aliens in Area 52, including two Daleks and the infamous Ro-man.
Gremlins. In a scene at a science fiction convention, Robby the Robot (from Forbidden Planet) and the time machine from the 1960 film version of The Time Machine appear. They both act the way they do in the films: Robby talks as he did in Forbidden Planet and the time machine can actually go through time.
Robbie the Robot has appeared in a bunch of stuff. He's even got his own IMDb page.
The pod from 2001: A Space Odyssey is visible in the junkyard scene in The Phantom Menace.
Young Frankenstein has the same lab equipment and lightning table apparatus props from the original Frankenstein movie - as in, they went and dug up the original props because they thought it'd be cool.
In Predator 2, one of the trophies aboard the Predator's ship as an Alien skull. That the two species inhabit the same universe would later become canon.
Orin Scrivello's dental "tools" from Little Shop of Horrors went on to appear as the plastic surgeon's tools in Tim Burton's Batman, and as gynecologist's instruments in Dead Ringers -- and according to some viewers also as implements of torture in both True Lies and Tomorrow Never Dies.

Live-Action TV

An episode of The Drew Carey Show focused on Drew winning the Batmobile from Batman and Batman Returns. At one point, Louis and Oswald are arrested for stealing the car, dressed as Adam West-style Batman and Burt Ward-style Robin.
Nathan Fillion took the catalyzer from Firefly and put it on the Castle set.
Babylon 5 had an unintentional example: They needed a space suit for the episode "Babylon Squared", and went to buy one from a prop company they dealt with. The only spacesuit available was the one from 2010: The Year We Make Contact.
There is piece of set dressing -- a desk-sized prop on wheels, on top of which are a pair of horizontal tubes with lights running through them -- which can be seen in dozens of SF TV shows and films dating back to the 1980s, including Airplane! 2, Babylon 5 and most of the Star Trek series starting with TNG. It's even been drawn into the background of one episode of Star Trek: Lower Decks!

Video Games

Duke Nukem Forever contains a lot of these, usually as part of Take That jabs at other shooters. For instance, at one level you find Master Chief's Power Armor from Halo; in others, you discover Dead Space Space Marine helmets (there is an achievement for finding all of them).
In the original Tomb Raider game, the Ark of the Covenant from Raiders of the Lost Ark can be seen in her mansion. This was of course removed from the remake, Tomb Raider: Anniversary.

Western Animation

The Simpsons: In a Halloween Episode, supervillain "The Collector" (Comic Book Guy) has the only working phaser ever produced - they made it to keep William Shatner in line.

Real Life

In Real Life, nearly any time David Hasselhoff appears in something just to show off, he'll be accompanied by K.I.T.T. K.I.T.T itself has often shown up in various works of fiction, mostly for Rule of Cool.