Google Single SignOn API

From their site: Security Assertion Markup Language (SAML) is an XML standard that allows secure web domains to exchange user authentication and authorization data. Using SAML, an online service provider can contact a separate online identity provider to authenticate users who are trying to access secure content. 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.