Taken from Slashdot:

"Nexuiz v1.0, an fast paced deathmatch FPS, has been released under the GPL. It uses a heavily modified version of the Quake 1 engine, with real time lighting/shadowing, improved particle effects and more. It looks really good and it's available for Windows and Linux, with a Mac version said to be coming soon."

http://games.slashdot.org/article.pl?sid=05/06/03/0649252&tid=112&tid=106

Game site is up at http://www.nexuiz.com

I've had a play around with it and it's great for a quick & dirty deathmatch. Kind of like old Quake DM. Plus it's free :D