Basically for viewing ginormous photos. They have a few samples like an aerial map of the world or a set from the Library of Congress (which is pretty cool... a lot tiny thumbnails come up and you can zoom into one with excruciating detail. Actually kind of neat. And free. You can also add your own images as long as they're hosted somewhere like on an RSS Feed, or other options are "Deep Zoom Content" or "Photosynth user", neither I'm familiar with. Gizmodo story here. App store link here.