Just figured I would register here to share with you the free FPS Xonotic.
Plays like a cross between UT and Q3, nice CPMA inspired movement etc.
For fans of arena FPS's it is really quite good and in constant development.
From the website xonotic.org
Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux. The project is geared towards providing addictive arena shooter gameplay which is all spawned and driven by the community itself. Xonotic is a direct successor of the Nexuiz project with years of development between them, and it aims to become the best possible open-source FPS (first-person-shooter) of its kind.
Short video here
(keep in mind that this is using a modified quake 1 engine!)
View release statement at http://www.xonotic.org/2011/09/xonotic-0-5-release/ (official mirror / torrent)
You can get the client from http://games.on.net/file/45012/Xonotic_0.5_Full_Game_Client