SocialAuth.NET
SocialAuth.NET is an open source library allowing authentication through Facebook, Yahoo, Google and MSN. Unlike many other pure OAuth libraries, this library not just provides authentication but also provides access to user's profile and friends list. A site with absolutely no authentication can be turned authentication enabled via aforesaid providers by merely adding few DLLs and making few Web.Config changes. SocialAuth.NET like Forms Authentication takes complete control of authentication pipeline and can also be used to compliment forms authentication.
Go to http://code.google.com/p/socialauth-net/
See also these related projects.