There is apparently a sport called "soccer" (sometimes confused with football) that is somewhat popular in various places around the world. For those who are particularly serious about getting the perfect kick, BallTune claims to be able to measure the pressure of a soccer ball simply by watching it bounce with your device's camera. Truly this is the future.
FIFA regulations require a ball to be inflated to between 8.5 and 15.6 PSI, but getting it just right can allow for better control. Different equipment manufacturers ship balls at different pressures, but BallTune lets you see where a particular one is just by bouncing it while pointing the camera that way. As the ball bounces, it eventually comes to a stop and the app uses the estimated range over time to determine the pressure.
BallTune claims accuracy within 1/16 of an inch as the ball bounces and 1 PSI. If you've got a soccer ball around, go ahead and give it a shot – the app is free.