.NET has been open sourced!
Note:
This post was written in 2014 and discusses technology that may be outdated.
The concepts may still be relevant, but specific tools/versions have changed.
Have your first look at Microsoft releasing the .NET source on GitHub: https://github.com/microsoft/dotnet
Full Linux and OS X support for .NET is coming soon! I hope this also heralds the arrival of Visual Studio for Linux and OS X :)