Seven or eight years ago, Microsoft donated the source code for a project I worked on to the open source community. The game was called Allegiance and it has been amazingly and admirably kept updated and enhanced by a group of volunteer testers, developers, designers and artists at FreeAllegiance.
When Microsoft released Kinect—a new system which lets people interact with the Xbox without a controller—a hacked version of the source code was posted on the Internet within a couple of weeks. See this Inc.com article. In the past Microsoft might have screamed, threatened, sued, or all of the above. Instead, they went to NPR and lauded the effort. Rapidly, people jumped on the bandwagon and have been enhancing the product.
We'll see if the interest is sustainable, but great move on Microsoft's part. Great to see some examples of openness!