Cube (OS X)

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.