Right now I’ve been playing Urban Terror (or UrT), it is a First Person Shooter (FPS) game similar to Counter-Strike. The game was developed using the open source Quake III Arena engine and can be played under GNU/Linux, Microsoft Windows and Mac OS X operating systems.
You can download Urban Terror binaries from Urban Terror Official Website
The game is available for 32bit and 64bit platform and is tested on Fedora 13 and Ubuntu 10.10 (Maverick Meerkat) releases