While Microsoft is certainly not an open source company there are some instances of code drops from, and open source projects started by teams at Microsoft that are released under licenses which comply with the definition of open source as defined by the Open Source Initiative (OSI). One well-known example is the Windows Installer XML (WiX) toolset, a more recent example is the source code for the additional Silverlight controls shipped during MIX which was released using the OSI-approved Microsoft Public License (Ms-PL).
The very latest addition to this list is a little project called "Transitionals" which - quoting from the project description - "[...] is a framework for building and using WPF transitions which provide an easy way to switch between UI views in a rich and animated way". The reason I wanted to highlight this project is that it was originally part of the incubation project "Acropolis" and while we are of course using the experience gained during the project for new technologies we are working on it is good to see that a small part of it has been released to the community in form of an open source project under the Ms-PL. Kudos to Jared Bienz for coming up with the idea and patiently pushing to make it happen (he documented the journey on his blog).
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