I just released a new version of an open-source SDK I've been working on to integrate the Stormpath authentication service into .NET projects. Check it out: https://github.com/stormpath/stormpath-sdk-dotnet
This release (0.2) added a bunch of functionality, including social login and authorization support. I blogged about the new features here: New .NET SDK Release: Social Login and Authorization
The library is written in C# and is totally OSS (under Apache). The authentication API service itself is free for small/hobby projects. (I work for the parent company)
I'm interested in any feedback! I'm pretty proud of the library itself. There are some tricky problems to solve, including LINQ to asynchronous data sources and implementing identity map behavior. Any critiques are appreciated. I'm on twitter as nbarbettini.