There's not many apps which make heavy use of the vibrating motor in an iPhone. Most apps will use it for notifications, that's it. But Cycloramic is an app which makes full use of what it has available. This app allows you to take panoramic pictures automatically. All you do is leave your iPhone up right on a smooth and flat surface, it'll then use the vibrating motor inside the phone to turn the phone automatically for you. This leads to a near perfect panoramic picture if you get the lighting right.
The automated function is fairly limited though. It only works on the iPhone 5. It's still pretty neat and is a good way to use a piece of hardware that's rarely use.