The venerable Quake may be getting old, but ID have polished up the old engine and released it as an in-browser free online first person shooter.
Quake Live is a beta version, and although extremely popular, there are always plenty of games to join. You have to register with a valid email address, but it's pretty painless. Once you're in, and have run the installer, you'll be taken through a really smooth tutorial. This is obligatory, partly as more Quake Live data will be downloaded as you play, and partly as it tests your skill so you get online games against similar players.
With the tutorial over, getting into a Quake Live game is easy. The maps are brilliant, it moves smoothly, the sound effects are perfect. The focus is on fun - it's not graphically violent, and many of the character models are quite silly - and it really is a breath of fresh air. The community features are good. You can find and invite friends easily, and join them in games. There are also extensive leader boards, to see how great you are compared to your friends, or the entire world.
It may look a bit old, and if you're a veteran, you might find the tutorial a chore, but it's such an enjoyable game to play, it's hard to hold this against Quake Live.