We've been working on a few projects for customers lately and one of the problems we've had to solve is Single Signon. It's a classic problem and I'm surprised that there are still a lot of well known sites that haven't solved it well. (For example, you're registered on their site and need to register again to access the forums.) The result is ClassyCAS. It's still a work in progress but we feel it's close enough to put into production. You can read more about it on the Econify Blog.
We're grateful that our customer allowed us to Open Source our efforts..