This is a sample .NET web site that uses Forms Authentication and Google Apps to authenticate the user. DotNetOpenAuth does the heavy lifting as far as OAuth goes. Most of the logic is in the login page at /login/default.aspx.
Thanks to Andrew Arnott (http://blog.nerdbank.net) for helping me understand a few things about DotNetOpenAuth on StackOverflow.
**** should add DotNetOpenAuth DLL to project