XAML Spy is commercial software. You can evaluate the fully functional product for 21 days. After the evaluation period you'll need to purchase a license, or uninstall the product. Once a license key has been purchased and entered, XAML Spy will work as a full version.
XAML Spy and Silverlight Spy can be installed safely side-by-side.
Looking for an older version of XAML Spy? Download links for older versions of XAML Spy are available in the release history document.
Modern UI for WPF, a set of controls and styles converting your WPF application into a great looking Modern UI app. Modern UI for WPF is an open source spin-off of XAML Spy. See http://mui.codeplex.com for more info.