The tool is very basic, it is a bookmarklet that takes you to the Google Rich Snippets testing tool but automatically fills in the URL of the page you are on, so you can save a step.
You can use it by dragging the following link to your browser favorites bar: Rich Snippets Testing Tool
It will save you a lot of time if you are the type of person who goes around the web looking at how your competitors or third party sites implement their rich snippets.
It is fun to see how sites use it, if they have coding errors or if they are using markups that might be worthy of reporting.
Forum discussion at Google+.