.NET framework: now with source code
Scott just announced it:
http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx
This will make debugging .NET apps a lot easier...
More details on how this is going to work exactly on Shawn's blog:
http://blogs.msdn.com/sburke/archive/2007/10/04/channel-9-video-more-details-on-reference-source.aspx
UPDATE: this is now available. See the following posts for details:
http://weblogs.asp.net/scottgu/archive/2008/01/16/net-framework-library-source-code-now-available.aspx
http://blogs.msdn.com/sburke/archive/2008/01/16/configuring-visual-studio-to-debug-net-framework-source-code.aspx