AssaultCube is a total conversion of Wouter van Oortmerssen's FPS called Cube.

Set in a realistic looking environment, gameplay is fast and arcade.
This game is all about team oriented multiplayer fun. Similar to counterstrike.

This package is in 'nonfree' because, although the Cube engine is Free, the
game data (models, textures, maps etc.) are non-free, preclude commercial
distribution, and in some cases do not allow modification.