I often find myself in situations where knowing a .NET class internal implementation would be very helpful. Will CollectionBase be performant for me? What internal data structure does it use? Why is my Regex expression so damn slow? How much overhead is my reflection loop incuring, and why?
When faced with that type of questions, you can often find an answer by peeking into the ROTOR codebase and seeing for yourself how things are implemented. There is a convenient online index at: http://dotnet.di.unipi.it/Content/sscli/docs/doxygen/fx/bcl/classes.html
Note that ROTOR includes only the framwork and the basic BCL objects - no ASP.NET, ADO.NET, etc. But it is still a fascinating resource for drilling down into the internals of .NET.
P.S. It seems that everyone on earth except me and Robert McLaws is at TechEd. So I guess it is up to us to keep the world economy running until everyone returns. No more coffee breaks!