This could be tremendously interesting: ShareID and Zopto. ShareID is a single sign-on and authentication system built around FOAF and the Atom API, while Zopto is the companion Social Networking system, again built around FOAF. All come from the dastardly mind of Ben Nolan.
I really like the idea of single sign-on, as anything that can facilitate an otherwise annoying process of authenticating. Also, I like the idea that I control my personal data by sharing a file sitting on my own site. ShareID also provides a log of one's activity, such as tracking comments one leaves on other sites (something I also wrote about in the same vein).
The only thing that I dislike is the idea of a third-party server reliance (and they already hint that the service will not be free for all).
I need to play with it to check if it passes mustard on this simple principle I already pointed out on comment authentication: the more extra steps it requires compared to good-old-way commenting, the more likely it will fail.