The Whidbey version of Rotor just went up for download on MSDN on Thursday. I downloaded it and built it this morning.

While some of the big rock features are getting press (e.g. generics, LCG, anonymous methods/closures, etc.), some of the smaller features are plenty cool, and can be grokked in their entirety in much less time. For example, do a 'grep -i nullable clr/src/*/*.*' if you want to see what went into implementing the Nullable DCR that Soma mentioned over here. And check out stuff like the WrapNonCompliantException function in vm/excep.cpp (and its callsites) to see how non-CLS exceptions get wrapped. And of course there's all that reliability work that went into Whidbey, leading to things like SafeHandles (vm/safehandle.cpp), and OOM and SO hardening.

Most of it's there for you to tinker with. Or to simply print out and enjoy, reading it as you sit beside the fireplace with a nice Bourdeaux. To each his (or her) own.