This application can measure your heart rate using one of two methods. One way is to place the iPhone, microphone-down, on the bottom of your wrist, or chest -- this method produced poor results.
The other method, which works well, is to use the standard Apple headset/mic combo and place the microphone portion on your neck. Both methods are sketchy in a noisy room, as Heart Monitor uses sound to detect your heart beat.
If you are in a noisy location, you can use the "Pulse Count" tab to measure your heart rate manually. The application will walk you through counting your pulse rate for 10 seconds. Then, you will be prompted to press the corresponding button for the number of beats. The application will then log your heart rate in the "Results" tab. It would be nice to be able to export the results somehow (possibly through email).
The hardest part to using this application is finding your pulse, but the developer has included pictures and tips to help. Overall, Heart Monitor works as advertised and has a beautiful user interface that gets right to the point. Be sure to look at our gallery of screenshots. You can purchase Heart Monitor for $4.99 on the iTunes App Store.