The open-source situation at Microsoft has really improved. Source is available for a lot of their web stack, contributions are accepted, and there are some nice open-source frameworks (like Nancy, MIT-licensed, inspired by Sinatra) that run on IIS or another Owin-compatible server these days.

I generally prefer Ruby-based things myself - this post isn't trying to sway anybody to the MS side; just wanted to give a bit of credit where it's due and perhaps a bit of news to those who haven't looked over onto that side of the fence in a long time.