Not sure they're releasing any OS code publicly though :-) internals still have to be somewhat guessed/reverse engineered at.

There are strings attached, but it is not that hard to get access to the Windows source code: http://www.microsoft.com/en-us/sharedsource/default.aspx

They've also added support for Docker, contributed code to Linux and are including a package manager in Windows 10. The world is turning upside down :)