This morning I took a stroll through the free bin in the App Store and found Earthscape [App Store link here] was, for a limited time, offered as a free download (it is now back to $4.99). I would pay a couple of bucks for Earthscape, although it isn't much more than a neat tech demo and didn't appear to serve any practical purpose. It is neat, but not essential.
The app is like a miniature version of Google Earth, minus street maps and not quite as "3D" or functional.
For a complete walkthrough of the app and its features, check the gallery.