Sauerbraten

3D game engine (more powerful redesign of the Cube engine) and FPS/RPG game

Experimental engine based on Cube, can be seen as "Next-Gen Cube", or "Cube 2". Much like cube, the aim of this engine is not to produce the most eyecandy possible, but rather allow map/geometry editing to be done dynamically in-game, and make map editing a lot of fun.