The sound of height
Welldrop lets you measure heights/depths using only a coin.
It uses the phone's mic to listen for sound of coin hitting the floor (which means the app works best in quiet environments).
When used correctly (see the guide below), it can measure with an accuracy close to 1/2" (1cm) for heights up to 3 meters. Welldrop is a handy tool for when a measuring tape isn't available or the height to measure is too high for tape.
Theory of operation is simple: you drop a small coin from 0.5cm (1/4") above the edge of your phone. Welldrop will measure the time between when the coin hits the phone and when it hits the ground. Using some simple physics magic, it calculates the drop height.
Welldrop shows the measured height in both metric and imperial systems. Just tap the figure to switch between the two.
Get source code for this app at https://github.com/oxplot/welldrop