.NET Framework Library Source Code

1. Download and install this QFE.
2. In Visual Studio goto Tools > Options > Debugging > General and turn off "Enable Just My Code" and turn on "Enable Source Server."
3. Go to Symbols and add this URL http://referencesource.microsoft.com/symbols and a local cache path. Make sure "search only when symbols are loaded manually" is checked.

http://www.hanselman.com/blog/NETFrameworkLibrarySourceCodeAvailableForViewing.aspx

http://weblogs.asp.net/scottgu/archive/2008/01/16/net-framework-library-source-code-now-available.aspx