From Eugene Siu's blog
Microsoft will open up source code of .Net Framework to the public. It allows outsiders to review what is under the hood, and enables easier debugging of development projects around .Net Framework. .Net Framework code has been reviewed heavily, and developers can pick up coding best practices by reviewing source code of .Net Framework.
The following libraries are available:
* Net Base Class Libraries (including System, System.IO, System.Collections, System.Configuration, System.Threading, System.Net, System.Security, System.Runtime, and System.Text).
* ASP.Net (System.Web).
* Windows Forms (System.Windows.Forms).
* ADO.NET (System.Data).
* XML (System.Xml).
* Windows Presentation Foundation (System.Windows)
Source code is made available under the Microsoft Reference License, which is intended for developers to have read-only access. However, it does not allow modification or distribution.
An additional feature: It supports JIT code download for debugging in the final release of Visual Studio 2008. If set up properly, you don't need to download all source code on your box. You can get it when you need it transparently.
You can read this article for more details.