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This is a free app designed to tell you your current heart rate. Of course it is not designed to replace medical attention but is more for fun and everyday monitoring if you are in relatively good health. How it works is you put your finger up to your camera while the app is loaded - holding it up to a light works best. The app will look at how the color of your finger changes because the blood pumping will indicate your heart rate. This is similar to how a medical pulse oximeter works.
I think this app is pretty accurate. I know my resting heart rate and it was close to that when I was relaxed. I did it after exercising and it showed my rate as much higher. I think it is a good app to check your heart rate during or after exercise or just out of curiosity from day to day. If you are having real heart problems then a doctor would definitely be in order though!
Image Source: http://itunes.apple.com/us/app/heart-rate-free/id401450171?mt=8