Hi,
a few weeks ago we released the source code of the Silverlight 2 beta 1 controls included in System.Windows.Controls.dll, System.Windows.Controls.Extended.dll, and System.Windows.Controls.Data.dll.
This is a very nice download to get a deep insight on how these controls are built and allows you to extend them to your needs.
The download also includes the unit tests for these controls, so you can start Test Driven Development with Silverlight.
The source code for the Silverlight 2 Beta 1 controls includes implementations for the following controls:
You can find the download here