Late post, but have you tried Team Fortress 2?
I've been playing a bit on there, and having a blast. It's a good change up from CoD: UO, which is the one I play most.
Then of course, you can download ZDoom, and hit doomworld.com up for a few (thousand) maps to play in. Never played online though. You'll need the Doom or Doom II IWADs, but ID still sells them online.