The Raspberry Pi Foundation has released OpenArena, an "adults only" multiplayer first person shooter based on the Quake III engine. Available now for Raspberry Pi systems, the game runs on a modified version of the engine that powers Quake III - id tech 3, and the gameplay has been heavily modified to work on the system. The copyrighted material from Quake III has also been removed to make the game truly open sourced. Obviously there are still some outstanding bugs in the software.
The Raspberry Pi store where the game can be downloaded seems to have its own ratings system and doesn't use the ESRB for its games. The game was rated "Adults Only" because it contains animated violence and blood.
You can download OpenArena here.
Source: Parity News