Microsoft just announced: The source code of the .net Framework (at least a big part of it) will be available shortly, as a standalone install or integrated in Visual Studio 2008. The code can only be viewed, not modified or distributed in any way, as it is published under the Microsoft Reference License.
Included is the source code for the .net Base Classes (System, System.IO, System.Collections, System.Configuration, System.Threading, System.Net, System.Security, System.Runtime, System.Text, etc), ASP.NET (System.Web), Windows Forms (System.Windows.Forms), ADO.NET (System.Data), XML (System.Xml), and WPF (System.Windows).
This will allow programmers to debug within the Framework, including better Call stack descriptions and Step Into MS’s code, so everyone can see what is happening behind the scenes. See ScottGu’s blog for detailed information and screenshots.