In case you haven't already heard from every other blogger (like Shawn, JasonZ and Brad) , V2.0 of the SSCLI (aka Rotor) is now available for download: http://msdn.microsoft.com/net/sscli. I just downloaded it myself.
This is basically the source for the CLR you know and love. Let me call attention to the debugging services (as that is the theme of my blog...):
I think this is very educational. I believe the source is valuable documentation, and you'll find that useful if you're crazy enough to be writing your own implementation of ICorDebug (like Roger) or even just consuming ICorDebug.