ScottGu has just blogged about the fact they they are making the source to the .NET Framework available so that you can step into it from your debugger.

http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx

I just love it how he and his team just stick this sorta cool stuff on their Blogs rather than going theough marketing (and LCA???)