http://www.xonotic.org/

I downloaded the game a few days ago. I knew (and played some) Nexuiz (the old one), but never heard of Xonotic, till TBs WTF... on the new Nexuiz, where he mentioned it.
After a couple of games I think it's a great game, although I'm not very good at it. ;-)
But when I'm in the mood for some FPS Action and don't want to play Tribes Ascend, then I join a server to get my rear end kicked. ;-)