BuddyPress.org

Yes. You login with your FB/Google/OpenID,Yahoo/AOL/MySpace account details.

You get a BuddyPress user account on the main/first blog of the site and can create a new blog which you are admin of. To log into that blog, you log into the main site and then navigate via the admin bar to your own blog dashboard.

To make it 100% compatible, it needs better integration with the BP Home template and ideally, being MU site-wide rather than installed in /plugins/ so that users can log directly into their own blogs rather than into the main blog and then navigate to their own blog dashboard.

For a drop in solution, I’m very impressed with it. And you get stats through using the RPX API key.