Here. "When I started looking at the source code for rotor, I couldn’t believe how invaluable it was. I also wanted to have an easy way of viewing classes in the files. So I wrote this little utility to help me out, to find common namespaces, classes, delegates, enums, etc in the source files."
This is an online browsable resource for digging into the source for a lot of the CLR base class library. Fabulous!
[JasonW or SamG -- it's hard to know]