AssaultCube
AssaultCube is a multiplayer first-person shooter game, based on the CUBE engine.
Features in a nut-shell:
Low latency, it can even run across a Lo-Fi connection
Features a single-player bot system.
Supports recording of your game by the "demo" system.
Contains 12 multiplayer game modes, including: Deathmatch, Survivor, Capture the Flag, Hunt the Flag, Keep the Flag, Pistol Frenzy, Last Swiss Standing & One-Shot One-Kill (plus team versions of these modes).
Comes pre-packaged with 26 different maps