Does anyone here play
Quake Live?
It's a 'beta' browser/plug-in-based version of Quake III Team Arena which you can play from within a supported browser (Firefox 1.5/2/3 and IE 6/7/8)
Ironically, as there's a ~350MB download (just over 320MB for the maps, and the rest for the browser plug-in), and a required plug-in to play, it's hardly portable.
-- And therefore defeats half the purpose of having it in-browser.
None-the-less, it's very fun to play, and completely free (thanks to in-game advertising similar to that which I've noticed in Counter-Strike -- which I paid for!).
EDIT: Also, if anyone wants an invite, please PM me your email address and I'll send one when I can.