An interesting app turns your iphone into a surprisingly accurate heart rate monitor. Instant Heart Rate can be downloaded in a free version with advertisements or a paid version without adds and with more features.
Basically you start up the app and place your index finger directly against the iPhone camera.
I have tried it on my iPhone 3GS and iPhone 4. The LED flash in the iPhone 4 gives you a faster and more accurate heart rate.
I assume the app must pick up the pulsations in my finger in terms of slight colour changes as the blood flows through. Pretty cool idea that actually works.