Using just the camera on your iPhone, the Cardiio app can accurately measure your heart rate. Here’s how it works:
Every time your heart beats, more blood is pumped into your face. This slight increase in blood volume causes more light to be absorbed, and hence less light is reflected from your face. Using sophisticated software, your iPhone’s front camera can track these tiny changes in reflected light that are not visible to the human eye and calculate your heart beat!
This video shows this process in action (with a short explanatory intro of the mathematical technique):
That is flat-out amazing. (via @delfuego)