As this affects (potentially) all aspects of development on the Microsoft .Net platform, whether it be adding plugings to Windows Live Writer, writing your own websites incorporating many of the Windows Live API's that have been published or maybe writing something that has absolutely nothing to do with Windows Live, I thought I'd give this a quick mention.
A few months ago, Microsoft stated that they would release the .Net Framework sourcecode and today they have followed through with that promise and actually released it. You will need one of the variations of Visual Studio 2008 to take advantage of this.
For more information please see Scott Guthrie's blog and Shawn Burke tells you exactly how to link your projects into the sourcecode and debug using it.