I've just released a first demonstration version of my 2D game engine and game creation toolset Duality. Both engine and toolset are written in C# and based on .Net 4.0 and OpenTK. Long story short: There is a demo video on youtube showing what you can do with the current toolset version, and how to do it and I'm collecting feedback.
If you want to test it yourself, you will need .Net 4.0 and Visual C# Express 2010 (or Visual Studio 2010). Here's the download. If you want more information on development or internals of Duality, you might want to check out my blog.
Any feedback is appreciated - comments, bug reports, feature requests, anything will help me improve future versions.