Aphrodite A from Mazinger Z, which uses a breast missile attack (much to the annoyance of its pilot, Sayaka Yumi). This example is the Trope Codifier, referenced or parodied in many of the entries below. (And traumatizing and/or intriguing viewers on either side of the Pacific for decades.) Diana A from the same series and Venus A (from Great Mazinger) use the same attack.
The Mazinger Angels spin-off manga gleefully Flanderizes this trope, using Aphrodite A and the other FemBots from the franchise and giving them an increasingly ridiculous array of weapons based on this trope.
Mahoromaticparodies the above with a scene where Mahoro fires breast missiles from a missile launcher disguised as a padded bra.
In Lupin III Alcatraz Connection, Lupin disguises himself as a woman during the opening, and squeezes sleeping gas out of his breats, knocking out the guards who were trying to chase him.
In RoboGeisha, Kikuyakko's first cybernetic upgrade is to have her right breast converted into a machine gun. (Her left breast? The trigger.) Additionally, the Tengun fire acidic breast milk from theirs.
The 'Power Pasties' in the soft-porn spoof Flesh Gordon shoot laser beams. Used for Rule of Funny when Gordon ends up wearing them.
The Ambushers, a Dean Martin "Matt Helm" vehicle, features a smaller caliber version of this in which each cup of a secret agent's bra is actually a gun.
An old joke tells of a woman who made a wish to a genie for her breasts to grow, and the genie made it so that they grow a size every time someone apologized to her (God knows why). The next day as she walked down the street, a man bumped into her, saying "Excuse me." Her breasts grew a size. Another woman bumped into her, saying, "Pardon me." The woman's breasts grew a size. Later in the day, she bumped into a Japanese gentleman who knocked her over. As she stood, the Japanese man bowed and said, "A thousand apologies." The head lines in the next day's paper read, "man killed by speeding torpedoes."
In Raymond Briggs' picture book "The Tin-Pot Foreign General and the Old Iron Woman" (an anvilicious allegory of the Falklands War) the Old Iron Woman has cannons in her breasts which fire when she is excited.
One of Timothy Zhan's Cobra novels features the first female Cobra, a group of cyborg soldiers. At their basic training, there is discussion of new weapons and upgrades, but ultimately these are not ready for deployment. One of the male recruits whispers something to his buddy that makes him laugh, and the instructor insists he repeat it so everyone can hear. The joke was that the female Cobra cadet could perhaps conceal an anti-aircraft battery in her chest (note that the character had not been described as particularly well-endowed.)
Live Action TV
Metal Alice, the one major female villain from Tensou Sentai Goseiger, can fire breast missiles in a similar vein to Aphrodite A.
In one of the many, many sketches on The Benny Hill Show, Benny is playing a secret agent in a trenchcoat. A sexy female spy corners him and shoots him — with breast guns. When these miss, Benny counterattacks by opening his trenchcoat and letting loose with hands-free machine gun fire.
In the video of "Firework" by Katy Perry, shots show fireworks coming from her chest area. (The implication is more that they're coming from her heart than her breasts, though, in line with the theme of the song - other people in the video, one of them male, also get fireworks shooting from their chests.)
Then there's the video for "California Gurls", which features whipped cream.
Given Demi's insistence that the rest of the party turn around while she installs it, and the attack animation when she uses it, her Phonomezer is likely one of these (although it fires sound waves).
In one strip of the beach vacation arc of Life of Maid, Flandre slaps Sakuya in the back, which ends up knocking one of Sakuya's pads out to hit Remilia in the face and triggering Sakuya's Berserk Button in the process.