At this point in time (early 2013) almost all the Microsoft web frameworks and libraries are distributed as open source software: ASP.NET MVC, ASP.NET Web Api, Entity Framework, Reactive Extensions, SignalR. You can contribute back to the original source or you can fork it and start developing and distributing your own flavour.
If you are a .NET web developer you are now using an open source development stack.

    On the application server front Windows Azure can boot up...