• http://openid.net/

 "a free and easy way to use a single digital identity across the Internet". 

• http://es.wikipedia.org/wiki/OpenID
• Un esquema de funcionamiento: http://www.windley.com/archives/2006/04/how_does_openid.shtml

• http://remysharp.com/2007/12/21/how-to-integrate-openid-as-your-login-system/
• http://qpleple.com/how-to-make-people-login-into-your-website-with-their-google-account/