A few newer phones like my Nexus 4 have a barometer to measure air pressure. Apparently this is used by the phone’s GPS system to determine your altitude to get a faster GPS lock.
I have seen a few apps that do something interesting with this data, one of these is pressureNET which crowd sources the data and displays it on a map (screenshot below). You can also get your own devices trend over time.
Anyone else find an interesting use for the barometer?