Capture That Flag
If you have Half-Life 2 for the PC, it's worth checking out the free beta of Half-Life 2: Capture the Flag. I ended up playing it with a few Ruffiani pals last night for hours and I wish I was still playing it now, in spite of the fact that the adrenalin is likely to send me into a flu relapse.
I should say here that I'm no CtF connoisseur, due in part to an aversion to public servers, so I can't really compare it to Unreal Tournament 2004 or whatever else is getting people to spend a thousand dollars on a video card, but I did have a lot of fun.