Altimeter puts the GPS capabilities of your iPhone to use as a handy altimeter. See your current altitude, set the base altitude before you begin a climb, run, hike, or bike ride, and then check again at the peak to see how far you have come. You don't have to have your iPhone on all the time, just turn it on whenever you want to check your progress. Altimeter remembers your starting altitude even when your phone is off.
A great tool, but of course it's only as accurate as the GPS readings that you are getting. It works best with clear views of the open sky, and won't work on devices that don't have full GPS hardware.