Blood frontier - http://www.bloodfrontier.com/ - Correct me if I'm wrong, but I think it's based of the Suaerbraten Engine. It's a decent game, worth a look, although quite unpopulated.
Nexuiz - http://www.alientrap.org/nexuiz/ - Originally I didn't like this one, but that was several years ago, and it's got alot more polish to it now. Definately worth a look as an alternative to Unreal Tournament. Lots of people on.
Sauerbraten - http://www.sauerbraten.org/ - Named after a type of food, the engine has gone through some heavy upgrades and tweaks since the early releases. Fast, old school quake-like, FPS. Lots of people on.
Tremulous - http://www.tremulous.net/ - Fast paced sci-fi FPSRTS. Play as Aliens or humans, each with their own strategy and play style. Definatly worth a look.
Urban Terror - http://www.urbanterror.net/news.php - Counter strike style game, with less hackers. Fun, lots of people online.
Warsow - http://www.warsow.net/ - Interesting sci-fi shooter with innovative graphics. Very movement based and pace oriented. Lots of people, worth a shot.
Open Arena - http://openarena.ws/ - Another open source online shooter. I found Open Arena a bit slow and cumbersome, wasn't much of a fan in the end, but there were certainly enough people online who enjoyed it. Check it out, might be your thing.
Wolfenstien Enemy Territory - I don't think there is a specific host for this game, so google it and you'll find it. As I remember it was a bit of a hassle to set up but that was a while ago, so I don't know if that's changed. Fun team based tactical goal oriented shooter, lots of people online. Worth a look.