I've been playing a LOT of UrbanTerror mostly because zog recommended it and it has AMD 64bit precompiled binaries available to play. This is a great game, the graphics arent so great, but as far as just get in there and start-shooting fix, its great. From the UrbanTerror site..

Urban Terror is a free multiplayer first person shooter, that does not require Quake III Arena. It is available for Windows, Linux and Macintosh. The current version is 4.1.
Urban Terror can be described as a Hollywood tactical shooter; somewhat realism based, but the motto is "fun over realism". This results in a very unique, enjoyable and addictive game.

Tonite i cracked the 200 points barrier which is great. Some tactical tips for UrbanTerror -

• My single biggest boost was learning the sprint keys, hold down forwards, left and right
• It's a fact most people don't play as a team and everyone goes for gold, learn to strategically place smoke-grenades around such places as your enemy's flag for CTF events, become a silent 'support unit', you may die a few more times but its great watchin your team totally nail the other side
• Develop a 'system' for capturing a flag, work out the best routes, and the average chances of attack from different parts of the map, rinse, wash, repeat and you'll get great results
• Some of the older-style weapons work fantastically!
• Aim for the head

On my Aspire 520 under Debian i'm getting roughly 90fps in 800x600, not bad!