id Software has started the public beta of Quake Live, this game is a free web-based version of Quake III. The game features five game modes and has dozens of maps, you can register over here to get access to the public beta.
QUAKE LIVE is a new and innovative multiplayer game experience from id Software - the creators of DOOM and QUAKE - that offers game players of all skill levels a totally free and easily accessible multiplayer game and community through a single website. Every element of the QUAKE LIVE experience, from finding and playing games to post match stats review and social networking are delivered through a single state-of-the-art website.
I tried it for a couple of moments and it's pretty good for a free game.