What the the Open source Library available in the C#.NET for the SSO.
Basically I want to connect with Google SSO then will further continue with other providers.
Check out: DotNetOpenAuth
Also, you may want to check the following links:
http://msdn.microsoft.com/en-us/library/ms972971.aspx
http://weblogs.asp.net/hernandl/archive/2004/06/09/ssoformsauth.aspx