Saw an app that does this on the android platform so thought I ‘d give it a go on iPhone.
It measures your heart rate by looking at the changes in skin colour as the blood flows in. It’s only available on the iPhone 4 as it needs the flash in torch mode to work.
You can get it here - it’s free! http://itunes.apple.com/us/app/heart-rate-free/id401450171?mt=8
I’ll do a blog post soon on how it works. It’s surprisingly simple.