SAML is the OASIS working group's network security architecture. It appears to cover authentication, authorization, and some flavor of directory service (e.g. user attribute publication). I don't know much about it yet, but i'm looking to learn more. This page is just random notes at the moment, and is not well-organized.
Among other services which allow the use of SAML, Google Apps allows it for authentication.
It seems similar to OpenID, but a bit more heavyweight.