Available now; the third preview of XAML Spy. Among others improvements include support for Silverlight 5.1.10411.0, property grid enhancements and new content view support.
XAML Spy is the visual runtime inspector for Silverlight, Windows Phone and WinRT XAML applications. XAML Spy is Silverlight Spy vNext.
Learn more and download the latest release at http://xamlspy.com.