http://www.quakelive.com/
Sign up bitches, should be just an email address. You can play Q3 for free on your browser or switch to full screen. It's pretty amazing considering you dont actually download the game, it's just a small browser plugin. But I guess the game is old enough to be open source and this is a project id software has been working on for a while.
If you can get past the first match with the bot, join me for some instagib! 1v1, railgun, 1 shot 1 kill, any takers? ;)