Mono isn't sponsored by Microsoft; they bought it. They also publish an insane number of things completely open source, developing them in the open. .NET Core

Mono isn't sponsored by Microsoft; they bought it. They also publish an insane number of things completely open source, developing them in the open. .NET Core being a huge one, but ASP.NET, VSCode, ChakraCore, TypeScript, and so many others. There are 2181 repositories in the MS Github account.