Firefox supports integrated domain authentication on windows. The big difference between it and other browsers like IE and Chrome is that in Firefox you have to whitelist every domain you'd like it to authenticate to.
It's also not a shipping browser, of course... yet.
Midori 0.1.2 ships with a copy of webkit that will render Acid3 100/100. Admittedly though, it is lacking in many other respects.
naugrim writes "Finally the reason for the long delay on the release of Gaim 2.0 has been made known... legal discussions with AOL. So now there won't be a Gaim 2.0, but instead a Pidgin 2.0. Gaim is dead! Long live Pidgin!"