Now we can debug the source code of .NET framework! This will really help in getting to the details of the libraries and developing a deeper understanding of things, and much less usage of the very famous Roeder's Reflector (www.aisto.com/roeder/dotnet/)
For more details: http://weblogs.asp.net/scottgu/archive/2008/01/16/net-framework-library-source-code-now-available.aspx