Now for something completely different - an infra-red remote control hijacker. By using an Arduino of almost any type, a 38 kHz IR receiver and a matching infra-red transmitter LED a lot of fun can be had. How? Simple - when the Arduino receives an command from any infra-red remote control - it transmits a random command to the units to be controlled. So if someone (for example) tries to change the TV channel, the volume might drop or some other random function be activated. Perfect for annoying people as a practical joke.
If you need to hide your Arduino board - size is important. And one of the smallest complete boards out there is the Freetronics LeoStick:
Apart from being one of the smallest Arduino-compatibles on the market with USB, it also has an onboard RGB LED and piezo which can be used a knock sensor and various tune and sound effects. Plus you can add extra circuitry with the matching protostick! For more information and to order, click here.