Last month there was an uproar over hosting Sandcastle, a closed-source tool, on the open source site CodePlex. In response to customer demands, Microsoft has released the source code for Sandcastle.
Sandcastle is the API documentation tool used internally by Microsoft for the .NET platform. It is being offered under the terms of the Microsoft Public License. This is a relaxed license that asks for little more than patent protection and attribution, making it suitable for integrating into build systems and the like.
You can download the source code at CodePlex.