Are there any cool free 3D games on the OS X platform? I've been recently playing Urban Terror, which is a multiplayer FPS game built on Quake 3 engine, and runs across Mac/Linux/Windows. Are there more such games that are

free
run on immersive 3D engine [not a Flash/HTML5 game]
belong to action-adventure/RPG/RTS/[insert your favorite] genre
possibly multi-platform [not must-have but a nice-to-have]

Thanks!
No discussion of free FPS games on the Macintosh can be complete without a link to Aleph One.

And of course, there's that recruiting tool, America's Army. Version 2.5 was available for Mac OS X. I guess version 3 isn't being developed for Macintosh yet…