If your computer isn't all that great, you might want to check out games like Tremulous or Wolfenstein: Enemy Territory. Both of them are based on the Quake 3 engine, and are quite a bit behind today's technology, but still have huge numbers of players. You can see what they're about at:
[URL=http://en.wikipedia.org/wiki/Tremulous] [1]
Tremulous[/URL]
[URL=http://en.wikipedia.org/wiki/Wolfenstein:_Enemy_Territory]Wolfenstein: [2] Enemy Territory[/URL]