Sauerbraten has an experimental branch called Tesseract

http://cubeengine.com/forum.php4?action=display_thread&t... http://www.youtube.com/watch?v=ZkFT1ra2w-A

Deferred shading, realtime lighting, etc. etc. Also it has the Sauerbraten in-game editor (like Unreal is getting).

Probably the best complete open source game engine out there. And it's zlib (unlike the id Techs which are GPL).