It's called exploDZ - Explore Deep Zoom - and its raison d'être is simply to help understand the structure and operation of DeepZoom images by 'exploring' them dynamically. Sure, you can read about DeepZoom and its file format, but my hope is that being able to visualize the layers and grid is more instructive (or at least a bit of fun)...
It looks like this
and should be able to parse and display any DZC/DZI/XML on the web - assuming the hosting site has a ClientAccessPolicy.xml (you can copy that one if needed).
At the very least, you can test it on these two collections: