The Source Code for the .NET Framework Libraries
One of the most useful tools to me while developing .NET code has always been Reflector by Lutz Roeder. Why? Well simply because it gave me a lot of insight into the .NET internals and how things really work instead of having to guess based upon the observed behavior. Now being an MVP gives me some privileges and extra information but we never got our hands on a copy of the source code 
.
Well it turns out that everyone, not just the insiders, soon will have that privilege. It appears that Microsoft is releasing the Source Code for the .NET Framework Libraries when Visual Studio 2008 and the .NET framework 3.5 is released. Not everything is released at once, some libraries will follow later. See Scott Guthrie blog post here for all the details.
Of course Reflector will still be useful to inspect other assemblies so I am not going to remove that excellent utility any time soon!
Enjoy!