Using my Droid X to measure my heart rate (Instant Heart Rate app)
There's a free app called "Instant Heart Rate." This app uses the camera and flash on your smartphone to measure your heart rate. You place your finger over the flash and camera and your fingertip lights up. The camera visualizes the pulsations in your fingertip and measures your heart rate.
I tried testing this app a few times on my Droid X and I was pleasantly surprised to find that it actually works quite well. The flash on the Droid X is so powerful that it will make your finger get warm.
Now I'm just waiting for someone to develop a pulse-ox...