One of the things my team has been working to enable has been the ability for .NET developers to download and browse the source code of the .NET Framework libraries, and to easily enable debugging support in them.
Today I'm excited to announce that we'll be providing this with the .NET 3.5 and VS 2008 release later this year.
Anyone who accepts the Microsoft Reference License will be able to browse and view the source code. The initial publication will include the BCL (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).
Read all about it here:
Here's a little more by Shawn Burke: