I just learned about Cosmos. An open source OS written almost entirely in C#. I haven't had a chance to really play with it, or even read that much about it, but the concept is intriguing, and I hope to get the time to play with it in the future. Along with the 10 million other things I want to waste my time one.
And yes I also learned of SharpOS at the same time, but since it uses the viral GPL. I refuse to link to it.