Singularity
Who said Microsoft is not into Opensource ?? :) :)
In the year 2006 (as far as i know .. ) Microsoft Research Team announced an opensource project - Singularity OS. When it was released, the functionality was similar to MS-DOS ..
Singularity was completely coded using C#.NET and the code is available for download.
To Know More about Singularity - Click Here
To start developing on Singularity - Click Here