This is the Snoop release (which I'm calling Snoop 2.5) which begins to support WPF 4.0.
So, now there is a 32-bit and 64-bit version for WPF 4.0 ... as well as the previously available 32-bit and 64-bit versions for WPF 3.5.
There is an installer for all four versions. I have set the default download to be the 32-bit version for WPF 3.5 ... but don't forget about the other three! Otherwise, you will have to build them yourself.
If you try and use the WPF 3.5 version to Snoop a WPF 4.0 application ... the Snoop app chooser can't find the application ... it will say 'Can't find an application, main window, or non-null PresentationSource root visual!'