Literal Junk Food

When someone takes his food from garbages and trash bags. This someone is usually either a human living in destitute poverty (Truth in Television), a stray animal, or an Extreme Omnivore.

Anime and Manga

 * Ganpachi Chabane, The Ultimate Teacher seems to eat only garbage, thanks to being mixed with cockroach genes.

Live Action TV

 * In an episode of Seinfeld, George eats an éclair that was in the trash. Unfortunately, his girlfriend's mother walks in while he's doing it.
 * It wasn't "in the trash". It was on top of the trash. Hovering. Like an angel.

Newspaper Comics

 * A Get Fuzzy comic strip had Rob discovering Satchel taking a piece of trash out of the garbage can and spraying liquidized cheese spray on it, so that it "tastes better". Naturally, Rob is disgusted.
 * Pooch Cafe: The dog Poncho is often shown eating out of trash cans.
 * Same thing in Buckles with the eponymous dog.
 * Garfield eats an apple from a trash bag in this Garfield strip.
 * He tried to invoke it in this earlier strip, but got disgusted by the smell of the trash can.

Video Games

 * In Earthbound, Ness could pick up hamburgers from trash cans early in the game.
 * The "bankrupt" screen from Monopoly for the NES and SNES.

Web Comics

 * In Freefall, Sam Starfall sometimes does this because his scavenger's digestion allows him to eat almost anything. He once even made shishkebabs out of garbage.

Western Animation

 * Raccoons and bears usually portrayed doing this, which is usually Truth In Television.
 * Many a Looney Tunes short begin with Sylvester looking through the trash as if it were a buffet, using a trashcan lid as a tray.
 * In the SpongeBob SquarePants episode "Just One Patty", Squidward is desperately trying to get a Krabby Patty without anyone finding out, going so far as to say that he'll even eat one off the garbage. Sure enough, a customer throws away his patty and Squidward dives right in. He ends up eating everything except the patty.
 * In Futurama, it's mentioned several times that Zoidberg eats garbage.
 * In the My Little Pony Friendship Is Magic episode "Applebuck Season", due to Mondegreening Pinkie Pie's requests for ingredients, Applejack makes a batch of muffins made out of chips, soda, lemon juice, and earthworms, which Pinkie nicknames the "Baked Bads". They hospitalized everyone who ate them, with the exception of Spike, who loved them. At the end of the episode, Applejack treats the other ponies to some apple juice, and Spike says he has the perfect thing to go with them: the Baked Bads, which Spike rescued from a dumpster.
 * From Tom and Jerry: The Movie, the duo's new friends Puggsy and Frankie da Flea offer them a buffet from a whole camaraderie of trash cans.
 * This has even showed up in a number of Tom and Jerry shorts.

Real Life:

 * The "Freegan" movement, which believes in not wasting food, no matter what. Dumpster diving is a big part of this.
 * Freegans are against waste in general, and dumpster diving for perfectly good stuff that was senselessly thrown away is part of the culture but not diving for food waste, usually. That's a bit more hardcore (and pointlessly dangerous) than most people are willing to accept.