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
The Windows Identity Foundation (WIF) is Microsoft's official library for identity federation.
In your scenario you can also use Access Control Service (ACS) to federate identity with Google, Yahoo!, etc. All of them (and more) are supported out of the box.
Samples, documentation, etc. available here: http://claimsid.codeplex.com
If you have a bit more control over your servers, Shibboleth might be an option as well. It actually simplified a large portion of our application that serves thousands of users and it is well thought out, flexible, and scalable. Basically, define a directory that should be watched for credentials and Shibboleth takes care of the rest.
You should use DotNetOpenAuth, as stated in Kamyar's answer. If you want to see a whole implementation using DotNetOpenAuth and other series of auxiliary libraries, you should have a look at this article:
Integrating OpenID in ASP.NET Web Forms using DotNetOpenAuth
来源：https://stackoverflow.com/questions/5977616/sso-library-on-asp-net-c-sharp