There's
Alien Arena.
I figure some of you guys would like it.
It's open source FPS, Q2 based.
It's got sort of a "Mars Attacks" theme going on. It's mostly DM, although there is a gamemode involving vehicular combat, and a really halfbaked CTF mode.
I haven't played it for a hell of a long time, but at the time, I absolutely hated the weapon "balance". Didn't run well for me either.
That was a long time back, though, and it might have improved. I'm not bored enough to check.
There's also 2 single player missions, although they are kind of bland, and one of them has a point where you can get stuck (unless they fixed it).
Nexuiz is super cool, though. It's another FPS, DarkPlaces based. It's linked to above. It used to be mostly DM, although now CTF has a pretty good hold. You probably aren't going to be able to get a good ping in North America, though, which can make the game stupidly frustrating since its version of the Railgun is rather popular.