Announcing the September 2013 Update (1.2.1.0) of XAML Spy. Among others this update adds support for Windows 8.1 RTM and Silverlight 5.1.20513.0, as well as support for the JetBrains dotPeek 1.1 decompiler.
For a complete list of all improvements and fixes of this September 2013 update, see the release notes.
The XAML Spy Power Tools extension for Visual Studio is unchanged, and works great with this update.