Some time ago, Peter Blois posted "Snoop", a tool which I find really useful for debugging WPF applications. It allows you to:
Browse the visual tree of running WPF applications
Inspect properties of elements at runtime
Edit properties of elements at runtime
Inspect RoutedEvents that are occurring, including the elements that handle them
Magnify sections of the UI
Find and debug binding errors
You may find more info and downloads on the Snoop official web page:
http://www.blois.us/Snoop/