When I took the first steps in WinForms, I learned from the source code of Paint.NET and several other applications (xacc ...) using WinForms.
Now I need quality code to learn about using WPF :)
Here is a couple I found useful:
Babysmash by Scott Hanselman is a study in his training WPF, and he has an excellent blog series when he moves from a Win32 approach to a WPF approach. And if you have children, this is a fun little application to smash them on the keyboard:
http://www.hanselman.com/babysmash/ - http://www.codeplex.com/babysmash -