.NET CLR Managed Runtime - https://github.com/dotnet/coreclr
.NET Framework - https://github.com/dotnet/corefx
.NET Compiler as a Service ("Roslyn") - https://github.com/dotnet/roslyn
.NET Orleans Actor Framework - https://github.com/dotnet/orleans
Mono Framework - https://github.com/mono/mono
Xamarin iOS, Watch, Mac Bindings and Framework - https://github.com/xamarin/xamarin-macios
Xamarin Android Bindings and Framework - https://github.com/xamarin/xamarin-android