Impossibly Low Neckline

Ah! How am I supposed to keep this thing on?!

—Van Von Hunter's sidekick dressing up as Lulu.

An off the shoulder neckline that is so low on the chest, it looks as though it's one deep breath from a having a Wardrobe Malfunction (but almost never does).

The actual outfit doesn't matter. It could be a bathing suit, a Spy Catsuit, a Mini Dress of Power, a Leotard of Power, a Sexy Santa Dress, or even a Pimped Out Dress, Fairytale Wedding Dress, or Happy Holidays Dress. It can also overlap with Victoria's Secret Compartment, even though it can stretch the logic of this trope even further.

This can be worn in Real Life and live action, but it always involves either a lot of adhesive, or sewing strips of plastic or metal into the seams to keep the shape.

A Sister Trope to Absolute Cleavage, Side Boob, Underboobs.

Compare Magic Skirt, Of Corsets Sexy, Theiss Titillation Theory, Impractically Fancy Outfit, Impossibly Cool Clothes, Form Fitting Wardrobe, Cleavage Window.

Examples:

Anime and Manga

In Rurouni Kenshin, Yumi's neckline was already pretty low to begin with, but it kept descending steadily until her kimono was more or less completely off the shoulders. Watsuki mentioned in a commentary that he heard from several cosplayers who remarked how impossible it was to maintain their modesty while dressed as her.
Ogin of Kyogoku Natsuhiko Kosetsu Hyaku Monogatari, when not in disguise, exclusively wears a very low kimono.
- Made all the more impressive by Ogin moving around more than most examples of this trope: she frequently shows off her ability to run, crawl, drag people away and even swim without falling out.
In ~xxxHoLic~, anything and everything ever worn by Yuuko. Then again, A Witch Did It.
Prome O and Nilval Nephew of Heroic Age. There are some shots where it seems Barbie Doll Anatomy may be the only thing keeping Prome looking even remotely decent...
Izumo no Okuni. Of course, given that she's also seen hovering in midair at times, she's probably levitating the kimono to hold it in place.
Belladonna, minor character in One Piece.
Sumire from Sakura Taisen.
Lust from Fullmetal Alchemist has both this and Absolute Cleavage.
Macross Frontier's Sheryl Nome is guilty of this in a few of her more elaborate pieces of wardrobe. Her outfit for Lion (the song) is particularly guilty. That said, it's completely justified - what she's actually wearing is a holographic bodysuit with her more physics-defying outfits projected over the top.
Of course Negima has this (un)covered. Mana infiltrates the Governor's ball in a disguise that includes a strapless, backless ballgown/corset. She's still wearing it during her Big Damn Heroes moment.
- She keeps a .50 rifle in that dress, exactly where you expect.
Black Butler II: Ran Mao's outfit at the Trancy costume ball in episode 5.
Mei Terumi, the Fifth Mizukage, in Naruto has a case so extreme that if it weren't for the chain-mail that she wears, we could all see her nipples.
Current time line Athena sports this in Hayate the Combat Butler.
In Magical Pokaan, the belt Liru wears as a top combines this with underboobs.
Ikaruga from Fairy Tail wears her kimono this way.
Alcyone/Alcion from Magic Knight Rayearth, specially in the anime. See here

Comic Books

In X-Men, the Hellfire Club outfits include bustiers that are like this, worn by Emma Frost, Selene, and Phoenix (and Jean Grey, depending on continuity). Some got even more ridiculous recently, although that's supposedly just Emma playing mind tricks... even though it doesn't explain any other impossible comic heroine costume.
Wonder Woman sometimes has this.
Gemini Storm's artist posted the inks for one of the pages. How are her breasts staying there?

Film

Jessica Rabbit from Who Framed Roger Rabbit.
Belle's yellow ball gown in Disney's Beauty and The Beast looks this way in some shots, but higher up in others.
- Her recent "Jeweled Princess" gown seems like this as well.
- Even in [[http://silverwingfox.deviantart.com/art/In-the-Black-for-the-Last-Time-215300222?q=gallery
Esmeralda also has a very low neckline as well, not just with her main Gypsy dress, but also with her poledancing dress and her white dress as well.
Meg's Masquerade dress and Christine's black dress in Joel Schumacher's Phantom of the Opera. Christine's neckline in the graveyard scene is highly anachronistic, since it's a friggin' "mourning dress", which were at the time of the film's setting supposed to be modest.
A lot of the women in Hammer Horror films would wear outfits that would hover just at that right spot to expose enough flesh to interest any vampires/werewolves/monsters passing by.

Literature

In the book Mrs. 'Arris Goes to Paris by Paul Gallico, this off the shoulder dress style is described as simulating the look of a naked woman under a sheet.
Used in The Wheel of Time when Nynaeve, Elayne, and their companions are traveling undercover as circus performers. Nynaeve and Birgitte wear dresses with Impossibly Low Necklines for their act, in which Birgitte outlines Nynaeve against a wall with arrows from a hundred paces. Played for Laughs because of Nynaeve's frustration with being forced to wear something so immodest, while Birgitte is having a grand old time of it.
In Jinx High by Mercedes Lackey, the villainess commissions a costume straight out of the American Revolutionary period for the school dance. The bodice is cut so low that one of her boyfriends has almost complete access to her boobs while she's wearing it (handy when you need to distract said boyfriend while the mind control spell takes effect).
Parodied in Gordon R Dickson's Hoka series: "pirate wench" Anne Bonney has to have a low neckline, but Hoka females are quadrimammarian. Her dress has two bodices.

Live Action TV

During the production of Star Trek the Original Series, costume designer William Ware Theiss was given explicit instructions by the network on how far he could go; one such instruction permitted costumes whose décolletage could expose anything all the way down to the top edge of the areola. Theiss, knowing a good thing when he saw it, followed those guidelines (ahem) explicitly.
Cynthia Watros in a couple of later episodes of Titus.
Some of the costumes on The Tudors.
Gossip Girls Serena van der Woodsen likes to dress like this.
In an episode of Living Single, Regine tries to describe a new dress to her friends, but they know her too well:

Max: Lemme guess, it's strapless...

Sinclaire: ...backless...

Kadijah: ...and hits the cleavage about here. (puts her hand about two-thirds down her chest)

Theater

Magda in Tanz Der Vampire. How is she supposed to scrub floors in that!?

Video Games

Lulu from Final Fantasy X. She keeps up her dress with a heavily boned corset and belts on the upper arms. But it's basically the fur trim that keeps her from total exposure.
- Van Von Hunter spoofed it in an Omake strip.
Amy in Soul Calibur IV also has a low neckline, with the fur trim being the only thing keeping the players from seeing everything.
The "witch" costume set from City of Heroes is obviously held up with ~~staples and glue~~ a NAILGUN!, because there's nothing else to keep it on your character's body.
Rouge from Sonic the Hedgehog. Might be justified by the fact a high neckline would be inconvenient for winged characters.
- And, to an (arguably) lesser extent, Marine the Raccoon. Hers isn't played for fanservice though - she's only seven, so there's nothing to show off anyway.
Morrigan from Darkstalkers has such a low-cut outfit that, if she wasn't a Horny Devil, would be a walking wardrobe malfunction. Of course, she's not actually wearing clothes...
Hanne Lichthammer from Clive Barkers Jericho wears an extremely tight-fitting, S&M-style SS uniform that just barely manages to contain her assets. Considering her horrifically mutilated appearance, though, this is definitely an example of Fan Disservice.
Sumire Kanzaki from Sakura Taisen, when wearing a kimono.
Jessica from Dragon Quest VIII features a gravity-defying shirt.
Several of The Sorceress' outfits in Gauntlet: Dark Legacy, but better displayed by the Blue Sorceress.
Kaguya Nanbu from Endless Frontier. The only thing that prevents her breasts from popping out of her clothes is the will of the Almighty, especially during bouncing.
Rose's new alternate costume in Super Street Fighter IV features this.
- One of her Street Fighter Alpha 3 winposes had her wearing a similar dress in pink.
Guild Wars starts off all female Elementalists in the Prophecies campaign in a shirt (though the application of that term is questionable) that can only be explained by magic.
Lotus, from Nine Hours Nine Persons Nine Doors has a really low bikini top.
The Disgaea series has archers, and Disgaea 2 has Rozalin.
Kai and Nou in Samurai Warriors.
Sona in League of Legends.
A male example in Inspector Grosky from the Professor Layton prequels. Just look.

Webcomics

Missi in this Misfile strip. Oddly, in the first panel, there appear to be (extremely thin) straps, but in the second panel, the trope applies. Not that there's much to fall out...
Dixie's outfit in What's New With Phil and Dixie after she's become Lawful Good combines this with an Absolute Cleavage. As she remarks, being übergood still gives you the fashion perks of evil.
It's justified in the 18th-century parts of The Dreamer due to it being in vogue.

Western Animation

In The Simpsons, Marge Simpson's default green dress. Once even lampshaded by a prison warden.

Warden: NOTHING IS KEEPING UP HER DRESS! ONLY HER MELONS!!!

Sneakily brought up in an episode of Superman the Animated Series where Clark and Lois are covering a Fashion Show:

Lois: "The only thing holding that dress up is faith!"