Warsow is an exclusively-multiplayer first person shooter based on a heavily modified version of the Quake 2 engine. Set in a cartoonish, futuristic universe, the game focuses on advanced movement strategies over straight combat (indeed, the game features a built-in "race" mode, which was the most popular mode at one point), although people do get shot at some point. Warsow is free and can be downloaded from its official site here; additionally, some Linux distributions feature it in their package manager. Ubuntu users can use this method.