Cosmos
Cosmos is an operating system "construction kit"
...Cosmos itself and the kernel routines are primarily written in C#, and thus the Cosmos name. Cosmos is not an operating system in the traditional sense, but instead, it is an "Operating System Development Kit". Cosmos lets you create operating systems just as Visual Studio and C# normally let you create applications. Most users can write and boot their own operating system in just a few minutes, all using Visual Studio.