The LiveJournal people have unveiled
their distributed identity system. Temporarily dubbed "yadis", it's based on FOAF and an AJAX-esque backchannel (though does not need to use AJAX technologies, or even JavaScript), and will ultimately allow sites to accept users authenticated on other sites; it also stands a good chance of somehow finding its way into the next version of SixApart's TypeKey.
Update: It has now been renamed "OpenID", and also has a nifty, and somewhat Apple-esque, grey-and-orange icon.
There's nothing AJAX-related as the backchannel. The demo I posted just happened to be of the AJAX flavor... but it can be done with old-school redirect-in-a-quick-circle as wlel.