I recently listen to a podcast by Brian Ellin and Doug Kaye on IT conversation. The topic of the day was a relatively new SaaS offering by JanRain. The service offers the ability to integrating existing authentication and identity services, for example, Facebook, Google, Windows Live, etc. Why ask your user to sign-up for yet another account when user identity is not part of your core business?
There were lots of attempts to contain the proliferation of user name, passwords and online accounts, but none that I know of really took the approach of integrating with existing services and then package it as a service. I think the guys at JanRain is onto something.
Check out RPX by JanRain @ https://rpxnow.com/