For in the Inner Geographer or Geologist in Us All

Columbia University's Lamont-Doherty Observatory has come out with a really neat app for the iPhone, iPod Touch, and iPad called EarthObserver.  It's currently free.  It can display rock formation information, USGS (United States Geologic Survey) maps, and tons of other information which I have not even scratched the surface of.
See:
http://www.earth-observer.org/ for more on this excellent app.