In The New Iteration, SNOOP is discussed, a great tool for visualizing and manipulating the WPF visual tree on the fly.  I also wanted to highlight some of the great community efforts that build upon this idea. Josh Smith, Andrew Smith and Karl Shifflett have released a Visual Studio Visualizer for WPF that is similar to Snoop but provides more features.

This product allows unlimited drilling into the visual and logical trees and any sub-objects of these, including objects in the managed heap. User can also view the run-time visual and XAML of the object.

The following two articles get into visualzers and explain the two versions of Mole.

Current Version:

http://www.codeproject.com/KB/WPF/moleIIforWPF.aspx

Original Version:

http://www.codeproject.com/KB/WPF/MoleForWPF.aspx

Mole's Home Page:

http://karlshifflett.wordpress.com/mole-visual-studio-visualizer-for-wpf/