Frankly I'm utterly astounded no one has mentioned this here yet, but I daresay most of you who read the Gamer Club forum regularly are already aware of it. Anyway, for those of you who aren't, ID Software have recently released their Quake Live game into public beta. Quake Live is basically full-blown quake...in your browser. It is fully multiplayer, and can also be played in a "practice" mode against the AI called Crush.
When you first sign up you'll have to install software from ID on your machine. Specifically, a somewhat bothersome Punk Buster service (I can't use Quake Live any more becuase Punk Buster is broken on my machine for some reason, it worked for a few days though). Once you restart your browser you'll have to wait for a half dozen files to downloaded into the browser before you can begin (another 40-odd will download over time, but only the first five are necessary to continue to the next step). Once the initial download is complete, your skill is graded based on your performance in a 1v1 first-to-15 match with Crush. Once that's over, you can begin taking on other players in the matches.
My personal experience with the game was reasonably positive. There were a number of crashes here and there (fewer and fewer toward the public beta release), and most notably the Punk Buster failure, but the game itself was absolutely excellent. Unfortunately, those of us living outside of the United States will have to tolerate higher ping times (on 9mb cable here in .au I was at around 230ms) but I managed to top my team a couple of times, so we can't be that disadvantaged.
Anyway, give it a try guys and let us know how you go