SAML Single Sign-On (SSO) Service for Google Apps

Google Apps offers a SAML-based Single Sign-On (SSO) service that 
provides partner companies with full control over the authorization and 
authentication of hosted user accounts that can access web-based 
applications like Gmail or Google Calendar. Using the SAML model, Google 
acts as the *service provider* and provides services such as Gmail and 
Partner Start Pages (PSP). Google partners act as *identity providers* 
and control usernames, passwords and other information used to identify, 
authenticate and authorize users for web applications that Google hosts.


http://code.google.com/apis/apps/sso/saml_reference_implementation.html