It allows you to spy/browse the visual tree of a running application (without the need for a debugger) ... and change properties ... amongst other things.
The most recent version of Snoop, Snoop 2.8.0, was released on October 4th, 2012. Go to the download page on CodePlex to download it. Most notably, with this release, Bailey Ling has added a PowerShell scripting tab.
The Snoop repository has been converted to Git and is now being hosted in two public repositories (which will be kept in sync), the one at CodePlex (http://snoopwpf.codeplex.com/) and the one at GitHub (https://github.com/cplotts/snoopwpf). See the 2.7.1 release notes for more info.
Documentation on How to Use Snoop
I am finally getting to business on the Documentation area on CodePlex. It will be a work in progress for a bit. Please forgive the mess.
Also, don't forget about the documentation on Pete Blois' Snoop page. It is still useful ... but hopefully will be less so once I finish my efforts.
Why Aren't My Apps Showing Up in the App Chooser?
One question that comes up all the time is the situation where the application you are trying to Snoop, isn't appearing in the application chooser (i.e. the combo box that lists the processes you can Snoop). This is more than likely a situation where the application you are trying to Snoop is running elevated (as Administrator). In order to Snoop these applications, you will also need to run Snoop elevated (as Administrator).
Snoop is not currently able to spy Silverlight applications (maybe some day). In the meantime, if you want to do that, I would point you to Koen Zwikstra's awesome utility, Silverlight Spy.