Conveniently Cellmates

People in prison end up having a cellmate who worked with them during their criminal career, despite the fact that no sane jailer would do something like that.

Examples of Conveniently Cellmates include:

Anime and Manga

In Tengen Toppa Gurren Lagann, Simon is jailed with Viral, The Dragon of the previous Big Bad.

Comic Books

A sequel comic to Superman Returns has Lex Luthor pull strings to get a less-than-bright cellmate, specifically so he can have an audience for his genius plans.
Just about anytime The Punisher ends up in prison, he runs into a hierarchy of prisoners who more or less run the place. For example, a Mafia family get huge, almost customised cells, with all the luxuries that can be brought in there, including TV sets, porno mags and blowjobs (from other inmates).
In Lucky Luke, the four Dalton Brothers will always get a cell together. Add to that the fact that it is a Cardboard Prison and they are all Tunnel Kings, and they will also very easily escape together.
The Beagle Boys (Disney) in various incarnations usually also get a cell together.

Film

Superman II: Lex Luthor and his henchman, Otis, are shown sharing the same cell in Metropolis Prison. They also work together in the prison laundry.
Batman and Robin. At the end, former partners Poison Ivy and Mr. Freeze are put in the same cell. Mr. Freeze promises to get revenge on Poison Ivy for trying to kill his wife.

Mr. Freeze: Surprise. I'm your new cellmate. And I've come to make your life a living hell. Prepare for a bitter harvest.

- Presumably there was some bribery involved in arranging the Batman and Robin example, considering that Gotham City prisons are unlikely to be co-ed.
In Goodfellas, Henry Hill shares the same prison accommodation as his gangster pals. Truth in Television for them and many other organized crime figures at the time, usually achieved through corrupt prison staff.
In My Cousin Vinny, Stan and Bill share a prison cell throughout the film.
The Usual Suspects: The main characters first meet in a jail cell, and this is revealed to be part of Keyser Soze's plot.
At the end of No Man's Land 4: Hot Rocks, the members of the all-female gang of jewel thieves are all locked up in the same cell.

Literature

When imprisoned in Malfoy Manor in Harry Potter and the Deathly Hallows, Harry and Ron are put in the same room as Luna. It does help that the 'prison' is basically the basement of Malfoy Manor.

Live-Action TV

In Oz, Ryan O'Reily arranges to have his brother Cyril become his cellmate so he can protect him.
In My Name Is Earl, Earl finds that he is cellmates with an old friend of his, Frank.

Western Animation

In Gargoyles, when the pack are in prison, Fox and Hyena are sharing a cell, as are Wolf and Jackal.
In the second Teenage Mutant Ninja Turtles 2003 cartoon, the four turtles get a cell together, when they get captured by the Triceratons. When Raphael wins a fight under the dinner, he is however in another cell alone, so the others have to free him when they escape.
In Futurama, Bender and Fry get implicated in a bank robbery with Roberto, a maniacal robot. Once they are imprisoned in the robot insane asylum, Fry's cellmate turns out to be... Roberto!

"I'm just practicing my stabbing motions!"

The Simpsons: Sideshow Bob has been seen with his family in jail.