The world is a change’n.
From the horses mouth, http://blogs.msdn.com/b/dotnet/archive/2014/11/12/net-core-is-open-source.aspx. They also have a GitHub repo, but it currently doesn’t include everything – https://github.com/dotnet/corefx.
This is very exciting news for .Net developers as this opens up our .Net skills to be eventually applied on other platforms (Linux, MacOS, iOS, Android…). Also, getting the opportunity to browse the code base and learn from it is appealing. Hopefully, I get a chance to contribute.