So, Microsoft is releasing the .NET Framework 3.5 source code when it releases the final version relatively soon here. You'll be able to be stepping through your code, jump into a method call, and find yourself browsing through Microsoft sources automatically downloaded from MSDN and matched up. This is pretty cool, and a welcome change. (It should be even easier than how I currently get framework source code on the corporate network.)
At first, I was skeptical that having the source code to something as big as the .NET Framework would be all that useful, and often, it's not. But, there are certain times when documentation is ambiguous and stepping into source code answers your question almost immediately. Having access to the source code by virtue of my Microsoftness has indeed been pretty helpful at least a couple times while working on ASP.NET features for SharePoint Designer and Expression Web. It won't answer all of your questions instantly, but I'm sure there are times when it will be a handy option, especially when you're dealing with "weird" things that are not usually documented too well, so it's cool that Microsoft is finally offering it to the world.