Zessica Wong from Aquarion Evol wears a short halterneck crop with a very large opening exposing her cleavage.
The titular heroine's outfit in Cutey Honey is a sleeveless unitard with a huge diamond shaped cutout that even shows off a bit of stomach. In Re: Cutey Honey she wears a halterneck crop top with a heart shaped hole over her cleavage.
Melissa's cleric dress in Rune Soldier Louie has a diamond shaped opening show off her cleavage. Genie's halterneck crop top has two opening, one on top of her cleavage and one on top of her Underboobs.
The Dirty Pair had these in some of their appearances, when the character design included collars that buttoned up but a large diamond shaped opening exposing their cleavages.
Arf's outfit in the first season of Magical Girl Lyrical Nanoha had a crop top with a diamond shaped keyhole. There's also Linith, Fate's Parental Substitute before the series started, who wore an outfit that comprised an outer-coat with buttoned collar and buttoned below her chest and dress with a diamond shaped cutout above her cleavage.
Ran in One Piece wears a crop top with a window hole in the middle.
In the first season of Galaxy Angel, Forte wears a dress with opening showing off most of her chest and Ranpha wears a Chinese inspired dress with a diamond shaped cutout above her cleavage. They were covered up in the second season onward for some reason.
In one instance of Death Note, model/actress Misa Amane attempts to get Matsuda out of a jam with a roomful of shady businessmen by pretending to want to be a model/spokesperson for them. She is seen with a tank top with a heart-shaped cut that revealed her cleavage
Medaka's student council outfit in Medaka Box has a huge opening on the chest area that shows off a lot of cleavage.
Amusingly, it becomes a major plot point because it violates the dress code.
Isabella from Beelzebub has a small opening in the chest area of her maid uniform.
A number of Robes in Mai-Otome cover their wearers from head-to-toe but leave opening to show off cleavage, mainly Maria's, Fiar's, Chie's second Robe, and Mai's. Akane's Robe just has it: only a little bit of fabric keeps it from Impossibly-Low Neckline. Oddly, Haruka's (for lack of a better term) off-duty uniform has a diamond shaped cleavage cutout, but not her Robe, as is the case with Una Shamrock in Mai-Otome 0~S.ifr~.
She eventually gets her iconic, plays-this-trope-straight, costume back after about a year.
One popular fan theory is that the window actually is a sort of Chest Insignia. If you look at it the right way it resembles the symbol on a computer's POWER button, you see.
Phantom Lady in 1946 had one of the first ones with her costume which was a mini dress with a plunging neckline and cape wrapped around her neck. Most of the other costumes merely had an Impossibly-Low Neckline.
Invisible Woman of Fantastic Four had one, shaped like the figure 4, in her Stripperiffic 1990s outfit. On more than one occasion has it been pointed out the physical improbability of maintaining the appearance of a number 4 in such a manner.
Rocket's first costume is a full bodysuit and a jacket covered her from head-to-toe but the bodysuit had a cleavage cutout, to Icon's vague disapproval. Her second costume, which Icon designed, eschewed this.
Taken to its extreme in Transmetropolitan #24. Channon Yarrow(an ex-stripper) wears a backless, strapless dress with an Impossibly-Low Neckline, sides slit to the hips, thigh-to-knee diagonal hemlines(opposing angles front and back), armpit length gloves, and no less than ten triangular windows; three on the outside of each glove, one on each side of her waist, one exposing her bellybutton, and one fully exposing her left nipple. She wears the triangle of fabric as a necklace.
Ra's al Ghul's daughter Talia often wears a gown with a cleavage window in the Batman comics.
In Origin Story, when Alex Harris (a Kryptonian trapped in the Marvel Universe) finally decides to stop running and become a superhero, she commissions a costume that is mostly an homage and tribute to Comic Book/Superman's look (and she feels unworthy to wear his colors, to be honest), but includes Power Girl's Cleavage Window because Alex knows the reasoning behind it and accepts it.
In The Wheel of Time Ebou Dari dresses are described as having an oval cut out exposing the inner slopes of the wearer's bosom. On married local women, this is to frame the marriage knife, who's hilt nestles between the breasts.
Another claim was that the armor was enchanted and either extended mystic protection to the open area, or that there was armor there that was made invisible.
A later novel in the series finally offered an explanation, saying that the armor was made by a vain sorceress who wanted to show off the goods, and did add a protection spell covering the "window" to keep it from being a liability.
The entire thing was lampshaded when Alias guest-starred in the Forgotten Realms comic book in 1989, and the halfling Foxilon Cardluck had the following exchange with her:
Foxy: Uhh... Doesn't that armor get drafty sometimes? Alias: It's enchanted. Foxy:(under his breath) Oh, I'll just bet it is.
From Star Trek: The Next Generation onward, the female Klingons are wearing outfits similar to their male counterparts, except with a cleavage-baring opening on the chest armor. Probably has something to do with the whole Space Viking motif. This is particularly silly, considering that Klingons attack each other with bladed weapons all the time. Even considering that Klingons have not one but two hearts.
Captain Liberty of The Tick has a star-shaped cutout on the chest of her leotard.
In season 2 of Babylon 5, Delenn experiments by exchanging her usual robes for a slinky black dress with a diamond-shaped opening over her cleavage. Everyone who sees her is stunned speechless.
The window is a not-uncommon part of Earth women's fashion - usually a high closed collar over a low neckline, creating the window. "Number One" (the head of the Mars Resistance) is one of the most notable bearers of such, wearing it in all her appearances.
Alexia from Pixelface has a bodysuit with a pentagon shaped cleavage cutout.
Pam from True Blood frequently boosts her cleavage with a corset, but only when she's in action mode do we not see this vampire bartender presenting a cleavage window.
Nicki Minaj in her "Turn Me On" music video wears a black Victorian style dress with a diamond shaped cleavage cutout.
Hyosung from Korean girl group Secret wore a white mini dress with a large cleavage cutout.
The female warriors of Laser War wear breast-separating jumpsuits that leave their centers exposed.
With some exceptions, almost every "Harness"-type body armor in Final Fantasy XI will have enclosing chest opening for both females and males. The defensive value of a Cleavage Window is hotly contested.
Fina's dress in Skies of Arcadia has a triangle shaped cutout in the chest area . It also has the distinction of a purpose outside of fanservice. The exposed skin is where Fina's shard of the Silver Moon Crystal appears.
Hsien-Ko from Darkstalkers wears an overtunic with a large oval shaped chest opening.
Norma in Tales of Legendia wears mini dress with a diamond shaped cutout at the center. She's also the least-endowed of the Legendia girls.
Orlha in Chrono Cross has mini dress with large diamond cutout in the center that shows off cleavage and some stomach.
In the first Master of Orion game, the Mrrshan ambassador has an halterneck outfit with a large keyhole in the front.
In Conduit 2, Andromeda's full-body armor covers everything except her face and the tops of her breasts.
In Ōkami, the first form/clothing set of Wood Sprite Sakuya, who is a peach tree spirit, has two Cleavage Windows. The first one is a diamond shaped cutout showing off her cleavage. The second ones bares her buttocks, to go even further then the first one in using her figure to remind a viewer of her connection to peaches.
Both the Vampire Armor and Royal Vampire Armor from Dawnguard have cleavage windows that conveniently only exist when worn by female characters.
Jihl Nabaat from Final Fantasy XIII and XIII-2 has an impressive cleavage window to go with her Baroness and Hot Librarian look. It's also where adornments such as dolls get placed when she is recruited as a Mon in XIII-2. In the sequels there's also Paddra Nsu-Yeul who has an open slit on the front of her top running from her chest down to her stomach.
Xenoblade has several sets of armor, with various holes cut over the cleavage, for Sharla, Melia, and Fiora.
Both Gold Jane and Iron Jane of the Jane Gang from Everyday Heroes wear leotards with teardrop shaped cutouts above their cutout.
Ferretina's, the Weasel Queen's outfit from this non-canon Girl Genius comic. She wears a torn crop top that not only shows off her Underboobs but also has a oval shaped cut out above her cleavage.
In Grrl Power, Halo warns Heavenly Sword that she'll be tagged as "Keyhole" if she doesn't introduce herself properly during her first appearance (even though it's a fight scene).
Sandra on the Rocks is a comedy about the fashion business, so it's not surprising if this feature is present on some of the flashier outfits seen in the comic — notably on Zoé's clubbing outfit here. Which may be a little unwise of her, given that she usually opts for "stylish", and this dress reminds us that she's got quite a bit of cleavage to show.
Lampshaded when discussing the design of Spinnerette's new suit, here.
Spinnerette: [...] If you could just give it a lower neckline with some cleavage[...] Sahira: What? Spinnerette: Y'know. To distract the bad guys. Sahira: You have a bullet-proof fabric, and you want to cut a hole in it right over your heart?! Are you insane?
Super MILF used to have one as her previously costume was a rip off of Power Girl. She apparently justifies this in that her "venusian bust" generates so much heat that she requires air venting. At least, that's what she says, though she also openly makes her opinion clear that the majority of costumed vigilantes do what they do mostly for sexual thrills, so...
It has since been shown that it generates enough heat that people standing close to her can feel it. Dressing modestly would at the very least be highly uncomfortable. Of course, that doesn't mean that she has to use a Cleavage Window. She currently has an Impossibly-Low Neckline instead.
Crimson, who is at the Superhero School Whateley Academy in the Whateley Universe. She has a diamond shaped window large enough that Phase thinks she's risking a wardrobe malfunction.
Galatea from the DCAU has an oval shaped opening on her white leotard; granted she's supposed to be an Expy of Power Girl.
The Futurama DVD commentaries mention that the animators liked putting Leela and Amy in dresses with these whenever the plot required them to dress up. Lampshaded in one episode that had random time skips during a basketball game. Amy, wearing such an outfit, immediately brings attention to it as a possible explanation.
In "The Devil's Hands are Idle Playthings," Leela wears a long formal dress with a keyhole in the chest to Fry's opera.
Izzy from Total Drama has one in her signature outfit. The top is a crop top with triangle shaped keyhole. In the second aftermath of Action, she uses hers to hold grapes, cheese and crackers, as well as the crumbs she spit out after realizing she was on the green room's TV.
In Stan Lee's Stripperella the title character changes her costume midway through the series. It looks similar to her old costume, the difference being her long sleeved crop top has a huge enclosed diamond shaped cutout.
On Steven Universe, Sugilite doesn't really have cleavage, but has a hole in her outfit to show off the Gem in her chest.
There are probably plenty of these at model shoots and show business events, but such an outfit can be very — notable anywhere else. In May 2012, a model named Julia Orayén was hired to present some materials to the candidates at the start of a Mexican presidential debate. On camera for less than 30 seconds, her tight dress with a large keyhole was an instant Twitter sensation, and news reports worldwide declared how she had upstaged — or even "won" — the debate.
A more mundane version is usually achieved by wearing a cardigan with only the top button fastened over an outfit that shows cleavage.