By googling for “Silverlight Snoop” (Snoop is a tool for investigating WPF GUI) I came (via jimmangaly.blogspot.com across an interesting tool: Silverlight Spy.
Make sure you have all prerequisites before installing it.
Not only it allows you to disassemble and show the composition of your own Silverlight application, but it also allows you to do it to any Silverlight application accessible on the Internet. All you need to do is to type in the url of the page containing that Silverlight application: e.g. I did it for my AWebPros banner typing in “http://awebpros.com/AWebProsMain.aspx” URL. Then I go to the panel on the right and expand “Silverlight 2 Beta 2 Application (Xaml1)”. Here is the resulting layout:
Silverlight Spy
As you can see, the TextBlock at the top is selected by selecting “textBlock” in the right hand panel!. When I try to select other text blocks, however, they are not shown in the right place – possibly because of the animation. Still it is a very impressive tool.
Nick Polyak's Software Blog 