As it was announced November 12, .NET Core will be open-source now
.NET Core is a foundation for future .NET platforms and it is used for by ASP.NET 5 and .NET Native. Microsoft is going to have cross-platform .NET and some community around it.
You probably know about Mono project with its separate code base, now it will be a part of this community:
But what is available on GitHub now?
Only small subset of libraries: Immutable Collections, SIMD, Assembly Metadata Reader, XDocument, XmlDocument and that's all. Everything else should be included there until Build 2015. Will be waiting for that!