.Net Framework Source Code
If you haven't heard the news yet, you can read the full article on ScottGu's Blog.
In summary Microsoft has released the source code for the .Net Framework in such a way that you can step into the Framework while debugging. IMHO this is great for users because it allows us to get a lot of insight into parts of the API that are tricky or not well documented.
Take a look at his blog for the full details of hooking this up into the debugger.