Source Code for the .NET
With the release of Visual Studio 2008, Microsoft will be releasing the source code for the .NET Framework Libraries.
The ability to be able to debug through your application and then right in to the framework itself will be a huge benefit for developers.
Initially the source code released will be for the following libraries;
* .NET Base Class Libraries (including System, System.Net, System.Security, etc.)
* ASP.NET (System.Web)
* Windows Forms (System.Windows.Forms)
* ADO.NET (System.Data)
* XML (System.XML)
* Windows Presentation Foundation aka WPF (System.Windows)
View the detailed info @ Releasing the Source Code for the .NET Framework Libraries - ScottGu's Blog