One of the nice things about being on the ASP.NET team is that I finally get to scratch the various itches (on that topic, Poison Oak...very itchy!) that have been bugging me for a few years. I finally get to play around with the source and see what effect changing various bits and bobs has on how stuff works.  I really do recommend spending some time spelunking in the .NET Source code. You can actually get this source (in a very non-official and non-supported way) using .NET Mass Downloader. This is just such an awesome resource...want to see how a server control like the Repeater works...look at the source! Trying to figure out why Viewstate behaves in a weird way...well, you get the idea. Right now you can get the source for these .NET 3.5 assemblies...that is a LOT of source!

WPF (UIAutomation.DLL, System.Windows.DLL, System.Printing.DLL, System.Speech.DLL, WindowsBase.DLL, WindowsFormsIntegration.DLL, Presentation.DLL, some others)

I don't know the official word on this yet but I'd be surprised if we don't continue adding to this list in future. From my team you can of course get the ASP.NET MVC source  already in Codeplex and we'll be adding even more projects to this site in the very near fact we now operate a 'why can't we release the source policy'...