I must say, one of the things I really like in Visual Studio 2008 now is the debugging with sources feature, allowing you to see the source of the .NET framework. Currently most of the base framework is available, and so is WinForms/WPF/... More to come.
I really like this because now I can learn more about the inner working of the framework and why my code is behaving in a certain fashion. Before you could also do this, using Reflector, but seeing the orginal sources with full local variable names is a lot more practical.
I do confess, when I am on the road the feature does require an active internet connection (because it checks to see if a newer version of the source is available, I think) and I do think it should work without a connection. Next version please :)
And it is soo easy to setup