The CLR Sources
I have no idea why this isn't in much wider circulation, but this is huge.
ScottGu has announced that Microsoft is Releasing the Source Code for the .NET Framework Libraries.
I am disappointed to see that even in the tiny source code samples that he has in the post I have violent disagreements (they speak about sealing stuff, which I have serious objection to).
This hopefully means a lot less ReflectorDebugging, although I am not sure about all the implication that this has.