It can be used to find your resting heart rate, track how your heart rate changes and check your heart rate immediately after training.
Because Heart Monitor uses sound to detect heart rates, you can use the iPhone 3G directly over your heart or directly on the pulse in your neck or wrist. You can get excellent results when using the microphone on your headphone cable.
Heart Rate Monitor application works best with iPhone 3G, works well with iPhone 1G/2G but you must use the headphone cable microphone, the inbuilt microphone does not detect heart beats.
You can purchase Heart Monitor for $4.99 via iTunes.