Andrew Robinson demonstrates PiFace Digital for the Raspberry Pi

Pi-Face Digital plugs directly onto the top of your Raspberry Pi, and allows you to sense and control the real world. Detect switch states, for example from a door sensor, a pressure pad or any number of other switch types. Then write your own software for Raspberry Pi that determines how to respond to that switch state. You can drive outputs to power motors, actuators, LEDs or anything you can imagine – including a twittering chicken!