Download Sauerbraten

Sauerbraten is an open-source major redesign of the Cube FPS engine with 3D floors, new weapons and better graphics. Like Cube, map/geometry editing can be done in-game. Sauerbraten has both multiplayer and single-player modes. There are two types of single-player mode, "SP" mode in which a player plays through a level or "DM", deathmatch mode in which you have to battle off respawning monsters.