I have just received a MakeyMakey today !
MakeyMakey is an Arduino based board that acts as a HID controller, in other words, as a keyboard or a mouse. It allows connecting whatever you like and control your computer with it. There are several funny experiments on the web.
My first experiment is a toy for the cat, the idea is simple: ground is connected to an aluminium foil the cat can stay on, and the space contact is connected to a rolled foil the cat can byte. Then I programmed a simple Processing sketch that just prints AUCH! when the space bar is pressed.
See the video here: