Altitude Alarm / Elevation Alarm
I was backcountry skiing yesterday. As we descended through the forest, it was critical that we started heading north at 9600' elevation. It would be awesome if there were some way to set an audio alarm on the smartphone for when I hit this elevation, so I didn't have to stop skiing every hundred meters of elevation to pull out my phone again and check it.
Altitude alarms are also useful for mountaineering and alpine climbing when, for example, you are descending a ridge line and know that you need to start rappelling (or hiking off the side of the ridge vs down it) at a specific elevation.