Navigation
GPS can be a bit ropey with altitude readings, and consumer versions certainly aren't accurate enough for working out which floor of a building you're on.
Some interesting stuff is being done combining GPS and barometers to allow for internal navigation of buildings/complexes*, as well as general improvement by making more data available (e.g. working out you're on the exit ramp of the flyover, rather than continuing along the motorway).
*Though GPS is of course quite likely to fall over inside most buildings with many floors of concrete, or surrounded by particularly leafy trees...