A free app for Android, by Utility Sabre.

Heart Rate Monitor is a handy app for Android smartphones. You don’t need any special equipment, just your own mobile phone with a built-in camera. All you need to do is cover the camera lens with one fingertip and stay still. It will take a few seconds to get the result, but you will get an accurate heart rate. You can use this app in the morning when you wake up, before you go to bed, and after workouts.

You will get a heart rate of between 60 and 100 bpm.

The app will work both in the background and the foreground.