When I was 13, I wanted to learn the drums. Since I couldn't bug my parents to get a me a real drum kit, I built my own setup with notebooks, hitting them and pretending to hear the sound in my head. If Makey Makey was around then, having to imagine those sounds wouldn't have been necessary.
A gadget that turns any real-world object into a touch-sensitive computer control, all I would have had to do is hook it up to a PC, attach the gator clips to my notebook drum set, run a percussion simulation app, and start hitting my fake skins. Yep, I could have built my own MIDI drum set out of nothing but some piles of spiral-bound lined paper.
Makey Makey is, basically, a hardware interface that turns any item attached to its clips into a replacement for keyboard controls. You can assign any key to each item: letters, numbers, function keys, direction keys and more. We're not sure if keyboard shortcuts can be programmed, too, but that would be fun: tapping a pair of scissors on your desk to perform a "Cut" operation, tapping on a piece of fruit to perform an alt-tab when the boss hovers nearby, or tapping on a toy in your desk to shut down your computer.
Each set consists of a circuit board that hooks up to your PC via USB and a set of alligator clips (you attach these to the board and to the object). The clips can either be clipped to objects or inserted into them (e.g. clay). The only requirement is that the object can conduct, at least, a small amount of electricity.
Makey Makey can run on any Windows, OS X and Linux machine. It's available now from Thinkgeek, priced at $49.99.