Over the last 24 hours, I've been playing around with the single sign-on (SSO) capability of Assistly. They have PHP and Ruby examples of how to do it, but I needed to get it working in C#. After many rounds of failure and impossible debugging, I finally got it working! Here's the code in case someone else wants to do the same. This is the code-behind file for a blank Login.aspx file using ASP.NET 4.0.