.NET source code available

Well, not all of it, but Microsoft has made it possible to enter the .NET Framework code when debugging. Although configuring Visual Studio to do this is a little more work than what you need to do with, say, Java and Eclipse (which is: none), I believe it is still quite a useful feature. It allows developers to go into the code of the .NET Framework and see what's happening with the objects that are passed on.
I have yet to test it, since I have Visual Studio 2005 and it's only available for VS2008, but I hope to get 2008 somewhere in the future.
Anyway, it's a little buzz on blogs here and there. Be sure to check out Scott Guthrie's post on it, where he mentions which namespaces are available for now, and which are planned.