When I first got my EVO 4G, one of the coolest apps I ran across was Instant Heart Rate, which measures your heart rate by having you place your finger on the rear camera lens. The drawback for me was that usually I wanted to know my heart rate while I was exercising and sweat had to be wiped off of the phone. So I didn't use it much, as cleaning a sweaty EVO is not one of my more enjoyed tasks.
What's My Heart Rate uses the same basic principle, but you can do it without sweating up the phone. It uses the front-facing camera to look at your face for micro-color changes as your heart beats.
In my testing, the app was usually within two BPM of what I was measuring on a watch, which probably means it is right on the money, or more than accurate enough if you're just trying to find out your heart rate on a treadmill, stationary bike, etc.
Unfortunately this app requires perfect lighting conditions and does not interface with apps such as Runtastic, but then again keeping a camera pointed at your face for an entire exercise session is probably not something that would work too well for most.