The Einstein's Pedometer app measures how much extra time you've gained by moving
Among other things, Einstein’s theory of special relativity says that as an object’s velocity increases, time as experienced by the object will slow down when compared to another object traveling at a lower velocity. This means that a 'relatively' short round trip on a space ship traveling at close to the speed of light would see you arrive home having aged less than those back on Earth. While the greater the velocities involved, the greater the effect, the theory applies to all relative movement. Now there’s an iPhone app that will let you know just how many extra nanoseconds you’ve gained by getting moving as opposed to sitting on your rear end.