One of My Favourites: Wolfenstein: Enemy Territory

Wolfenstein: Enemy Territory (or ET for short) is a FPS (First Person Shooter) game, native for Linux, which borrows concepts from Return to Castle Wolfenstein, and uses the Quake 3 engine. The idea initially was to develop it as a new commercial RtCW mod, but, ultimately, it was released as a free standalone, multiplayer game, after the developers abandoned the development of the single player part of the game.