Sauerbraten AKA Cube 2 SVN update

Sauerbraten is a FPS game with a real decent engine that is known to have a real good ingame map editor which you can even do team map editing online with friends, allowing for rapid development with help from your friends. It has ragdoll physics, a good single player side, and is updated almost constantly. This last svn update added a slew of new weapons and content you can check out. (Click for their website here)