ETH Zurich open-sourced an experimental operating system called BarrelFish, which it built in conjunction with Microsoft Research. The purpose of BarrelFish is to explore the best ways to structure OSes for multicore systems.
BarrelFish is a completely new OS "built from scratch," so it doesn't depend on any proprietary Microsoft components. The source code is available under an MIT license.
The BarrelFish site actually runs on a BarrelFish server, but the OS is a long way from being used for mainstream applications. Still, it's a step forward for Microsoft to be working on an open source operating system, even if it is "just" Microsoft Research.
I've made the case that Microsoft needs to open-source something big. I wouldn't count this as "something big" yet, but it is progress.
One criticism I frequently hear of Microsoft, and the Research division in particular, is that it works on many cool projects that are ultimately abandoned and never released. Making work open source means that even if Microsoft decides to pull the plug on a project, other developers can continue the work if there's enough interest.