I just downloaded and used a new iPhone app called Cardiio that measures your heart rate with the front camera of your iPhone.
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!
It works amazingly well, keeps track of your various measurements over time and shows you how you’re doing relative to others your age.
This is really incredible and I can’t wait to use it on a hike to see what my heart rate is on a steep climb.