I like this app more than I probably should. It isn't especially fun or useful to me, but every time I open open it, I feel so very impressed with the current state of technology.
Instant Heart Rate is an app that uses your phone's camera (and the flash if available) to determine you current heart pulse rate. Growing up I never imagined phones would be capable of such craziness! You put your finger on the camera lens and the flash lights up to illuminate the blood vessels in your finger. The app then monitors the expansion and contraction of those tiny blood vessels to determine your pulse.
It seems to be pretty accurate too, because resting, my pulse is usually in the 50s, after working out its around 100, and after 8 cups of coffee it was 106, and all of those figures seem about right to me. This could be useful if you're in the medical field or if you just want to show off a cool capability of your phone.
Also, they make flashlight apps, but this turns the camera flash on continuously, so you could use this instead of a separate flashlight app and have the much cooler option to check your pulse whenever you want. It even includes the option to share your pulse on facebook, blogger, and other sites right from the app. Oh, did I mention it's free?
Screenshot of my current pulse:
And the QR code: