http://www.cubeengine.com/

Cube is an open source multiplayer and singleplayer first person shooter game built on an entirely new and very unconventional engine. Cube is a landscape-style engine that pretends to be an indoor FPS engine, which combines very high precision dynamic occlusion culling with a form of geometric mipmapping on the whole world for dynamic LOD for configurable fps & graphic detail on most machines. Uses OpenGL & SDL.

havnt tried it but could be worth watching

Looks like a cross between Serious Sam & Doom2...But it does also look like a lot of fun! :)

Played this about 6 months ago, i didnt like it...