Microsoft has this week announced that it is providing the full .NET Server stack into Open Source. This includes ASP.NET, the .NET compiler, the .NET Core Runtime including the Framework and Libraries. This Microsoft says this should enable "developers to build with .NET across Windows, Mac or Linux". For the details, see http://go.microsoft.com/fwlink/?LinkId=518340.
And as if that were not enough Open Source goodness, Microsoft also announced Visual Studio Community 2013 – a free, fully featured version of Visual Studio including full extensibility. Developers, or anyone else interested, can get started with Visual Studio Community 2013 HERE.