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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 Start Pages. 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. There are a number of existing open source and commercial identity provider solutions that can help you implement SSO with Google Apps.