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 PLF non-free 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.