They are also open sourcing the chassis management system that they use for these systems.

They open sourced the hyper-v drivers for linux and freebsd and got them integrated into the kernel.

Asp.Net is completely open source and hosted on GitHub.

Here's a list of open source projects that they have started or contributed to on platforms/languages like Java, Node.js, PHP, and Ruby...

http://www.microsoft.com/opensource/directory.aspx

They open source quite a bit of software...