Quake Live!

[Laggy]

Quake! (http://www.quakelive.com) man this is fun, and free! you guys should try it out!

[Ripskin]

I played in the Beta since Quakecon last year, was fun for a few hours. Never was a huge fan of Quake 3 and its just the same thing from a browser.

Instant Action has some Tribes action on its browser based game. Little laggy last I played though.