There's a few other opensource FPS's that I keep an eye out on, Alien Arena recently did a new release. This prompted me to have a look at the other free FPS's that I occassionally play and or download to look at to see if its gotten any better.

• Alien Arena
• Urban Terror
• Nexuiz
• World Of Padman
• Warsow
• Open Arena
• Sauerbrauten
• Urban Terror
• Digital Paint
• Tremulous

The games all vary in community size and quality, I find that Urban Terror is probably the best to play, it's very CounterStrike like and people tend to pick it up quicker than the other games which tend to be pretty wacked out looking. I am a bit partial to the look and feel of Warsow, its a nice alternative to Open Arena or even Quake Arena for deathmatch games. World Of Padman and Alien Arena might be suitable for the younger players or more casual gamers.

One important note about the listed games, they all have windows, linux and osx distributions so everyone can play!