Today is a huge day for .NET! We’re happy to announce that .NET Core will be open source, including the runtime as well as the framework libraries.

This is a natural progression of our open source efforts, which already covers the managed compilers (C#, VB, and F#) as well as ASP.NET:
C# & Visual Basic ("Roslyn")
Visual F# Tools
Entity Framework

This takes it to the next level by extending it to the .NET runtime and the core framework.
.NET Core Framework

A Guy