Android: If modern sensor technology has taught us one semi-creepy thing, it's that reading your heart rate is stupidly easy. Now, Android users can join the action with Instant Heart Rate.
To use the app, you place your finger over the camera. It will then turn on your camera's LED to illuminate your finger and allow it to sense your heart rate. While it can't be guaranteed to be as accurate as proper medical tools (and should not be used as such), it can do a down and dirty job to get you in the ballpark.