Although it was once an avowed enemy of open source software in general and Linux in particular, Microsoft has developed its own version of Linux. Called Azure Cloud Switch (ACS), this distribution was designed specifically to help Microsoft manage its data center network. Microsoft describes it as "a cross-platform modular operating system for data center networking built on Linux."
ACS isn't publicly available, and it isn't clear exactly what Microsoft plans to do with the technology. Some speculate that the company will use it to add value to its Azure cloud computing services. In any event, it's an interesting sign of how much Microsoft's attitudes towards open source software have changed.