I found a nice post related to Single Sign on.
Here is another one with member ship API
Single sign-on across multiple applications in ASP.NET
Single sign-on with Forms Authentication
Implementing Single Sign-On and Sign-Out in ASP.NET Apps
Single Sign On across multiple ASP.NET applications
Understanding Single Sign-On in ASP.NET 2.0
You can download sample code for this at Single Sign-On Enterprise Security for Web Applications
Here is the one with example Single Sign on using ASP.Net (C#.Net)
Hope this helps!