Mac: Pausing and restarting music isn't exactly the hardest thing to do, but if you find yourself away from your chair, you have to move to the keyboard or mouse to stop playback. Flutter is a Mac app that allows you to pause and restart your music in iTunes with a hand gesture.
Flutter uses your webcam to track your hand gestures. The same gesture starts and stops your music, so if you put up the palm of your hand, Flutter recognises it and pauses the music as long as you're within about 2m of the camera. This does mean your webcam is on all day long and staring at you, but Flutter insists it doesn't collect or store that information. Flutter is still in its early stages, and the pause and start gesture is the only one currently available. It's a free download.