This is huge. Actually, not huge but still. 
Scott Guthrie today posted that Microsoft will be opening up the source code of some parts of .Net framework. And the plan is to keep doing it for rest of the libraries.
Its not like we can’t browse through the .Net framework source code now but the really nice thing about this move is that you can step through the actual .Net code while debugging your source code.
So, you are on a breakpoint on .Net method call in your code and if you hit F11, you will land right inside the .Net framework code. I can’t remember how many times I went back to Reflector to find what exactly was going on. This is going to help everyone, big time. Even Microsoft will gain something in this. I bet they will cut a lot on support calls.
Now, don’t confuse this with Open-Source as lots of people are. This is NOT Open-Source. The source is released under Microsoft Reference License. As the name suggests, this is just for your reference to help you understand .Net better (if you opt to) and allow to step through .Net source while debugging.
Amazing decision. Development will be fun now. I mean, more fun. 
Enjoy!
