Phosphors is browser-based and on LAN and free internet play
WarRock is free
Quake Live (watch to see if it goes public (current private) beta, its Quake 3 in-browser)
Quake Live Alternative
Fallen Empire: Legions is good, free, browser and popular but I don't trust the site very much. (Owned by a parent company, same as Ask Jeeves, and hosts some spyware.... but its worth it probably because I'm only like 10% on that particular site having spyware..I'm done now..)