Looks like you are looking for Silverlight Spy. Want to learn more about its successor, XAML Spy?
XAML Spy is the visual runtime inspector for Silverlight, Windows Phone, WinRT and WPF applications. XAML Spy is the successor of Silverlight Spy, it uses and extends the Silverlight Spy technology and adds support for new platforms.
See the overview of new features and improvements to learn more.
Are you a licensed Silverlight Spy user? Learn more about how you can upgrade from Silverlight Spy to XAML Spy. In many cases, upgrading to XAML Spy is free!
Not only does XAML Spy provide more features and support more platforms, it is available at a lower price point! See the XAML Spy purchase page for more details.