I've just been playing Open Arena, an open source FPS based on the Quake engine that's free to download. It's great stuff for half an hour's mindless blasting. Some real good maps and weapons too. Plus, it'll run on practically any old machine and supports Windows, Linus and Mac OS, so anyone can play. Did I mention it's free? Check it out here http://www.openarena.ws/?about
There's another one called Nexuiz, which needs a bit more clout to run it which I might try downloading. My machine hasn't got a great graphics card, but apparently you can turn a lot of the processor hungry elements to keep everything zipping along. There's a link to the Nexuiz site on the Open Arena site. I'm off to get my ass kicked again.