Fake Boobs

Eris pads her chest.

—Aqua, Kono Subarashii Sekai ni Shukufuku o!

Some women are concerned with their cup sizes, so they try to increase them with padding or surgery. Usually used as a way to demonstrate A-Cup Angst, and as a method for Breast Expansion.

For Crossdressers, simply using appropriately sized balls will suffice.

Anime and Manga

Eris in Kono Subarashii Sekai ni Shukufuku o! wears pads. Something Aqua freely shared with a worshipper that feels troubled when he found himself lusting after Eris.
Played with in Kuragehime: when Wholesome Crossdresser Kuranosuke passes out in the lobby of their apartment building, Mayaya attempts to help by applying Vicks Vapo-Rub to "her" chest, and in the process discovers that Kuranosuke's breasts are fake. Mayaya and the other members of the Sisterhood present completely miss the obvious and instead accuse Kuranosuke of padding "her" bra to hide her A-Cup Angst.
Used in Toradora!; Taiga isn't just very short, but a severely late bloomer. Ryuji helps her out by creating padded inserts for her swimsuit, which would otherwise make her lack way too obvious.
Great Teacher Onizuka: in the final chapter, the newest teacher is discovered (through actual groping by a female student) that she wears a padded bra.
In Pokémon Special, Blue infamously once hid two Pokéballs in her dress. She then taunted Sabrina into ripping her top off which released the Pokémon inside.
The Substitute Queen of Kamabakka Kingdom in the One Piece anime filler wear those under his dress.
Yun-Yun from Canaan sometimes stuffs Dumplings in her shirt.
Whilst she's not completely lacking, Karina Lyle of Tiger and Bunny wears a padded costume as the superheroine Blue Rose. Her sponsors are marketing her for her sex appeal (much to her annoyance), and are rather insistent on the importance of the Most Common Superpower.
James/Kojirou of Team Rocket, in one of Pokémon's most infamous episodes ever, wore a pair of inflatable breasts (and, thank gods, a towel) to a beauty contest. The audience was very impressed.
Kamome in Cyber Team in Akihabara wants to have bigger breasts for Gold Digger reasons and consequently stuffs her bra to appear bustier than she really is.

Comic Books

In Le Petit Spirou, little Suzette is once seen fitting tangerines in her tank top in a ladie's bathroom to see how she'll look when she's all grown up.
Ninjette once used them to impersonate Empowered using kyonyujutsu (fake boobery magic).

Film

There's a whole scene in Wild Wild West where Jim is telling Artie that his fake breasts don't feel right because they're too firm and don't have any give. He dumps out whatever Artie used for filling and uses warm water instead.
In the movie Dead Presidents, the protagonist sleeps with his girlfriend the night before he goes to the Vietnam War. As he takes off her shirt, he notices that she has her bra stuffed with tissue but tells her he doesn't care.
In Porky's II: The Next Day, part of the plot to take down the corrupt government man involved fake boobs full of cream corn and vegetable soup that the girl opened up and emptied as fake vomit.

Literature

In Girl, Fifteen, Charming but Insane, Jess stuffs her bra with plastic bags filled with minestrone soup. Hilarity Ensues when they're popped.
Lisbeth Salander, the famed girl with the dragon tattoo, uses these towards the end of that novel while pulling a heist. She keeps them afterwards.

Live Action TV

In Just Shoot Me, Nina is offered a free procedure from a plastic surgeon and is considering breast augmentation. So she goes around wearing D-cup inserts to get a feel of them.
On one episode of Will and Grace, Grace is worried that a newspaper photo of her makes her look more endowed than she actually is, so she goes to a gallery show wearing a water-filled bra. Unfortunately, it springs a leak.
In an episode of The Wonder Years, a legendarily endowed cheerleader is revealed to have stuffed her bra, leading to the characters being very disappointed.
An episode of The Fresh Prince of Bel-Air concerns Ashley stuffing her bra at school to get more attention.
The Cosby Show: Vanessa wants to make a sing-and-dance video at the mall. Claire pulls the stuffing out of her bra.
In one episode of Clarissa Explains It All, Ferguson reveals that Clarissa stuffs her bra.
In one episode of Popular, a pneumatic enhancement bra winds up as (in the principal's words) "a Hindenburg-like lingerie disaster".
In an episode of Little House On the Prairie, the adult Laura recounts an incident of trying to stuff three apples down the front of her dress, with predictable results.
Part of Jan's character arc/breakdown on the US version of The Office. Several episodes after Michael describes her chest as "nothing to write home about" and "shrunken chesticles," Jan gets a breast augmentation. The actress doesn't--on the DVD commentary she complains (jokingly) about the push-up rig and days when she couldn't wait to get out of it.

Web Comics

Double K: Simon uses baseballs in his disguises.
Megatokyo: Kimiko was seen using pads at Anna Miller's.
Zoe of Venus Envy, being a teenage transgender woman, uses little beanbag thingies to stuff her bra. (While she does take hormones, they haven't given her much of a bust yet.) At one point this is discovered by her soccer teammates, but they just assume she's embarrassed about being flat-chested.

Western Animation

The Venture Brothers: Dean discovers this of his Distaff Counterpart, but it's unclear from his reaction whether or not said character is just padding, or is cross-dressing.
South Park's Wendy, at the end of "Bebe's Boobs Destroy Society." She's thoroughly mocked for it (she's a fourth grader!) and is back to normal by her next appearance.