Makey Makey". It's a freakishly cool device that allows you to plug nearly anything that can conduct even small amounts of electricity (ie. potato) into your computer and use it as a controller. No for real.
At a mere 40 bucks a pop, every designer can have one of their own to take plugging away wildly into the artifacts of their lives (pets not recommended) for a taste of some pretty impressively embedded computing.
Joking aside, I can't think of a better prototyping tool for handheld control devices. You can mold that clay on the spot, try it again, adjust for ergonomics and continue your game of pong. Moreover, what an incredible way for kids of all ages to engage in learning. Thanks for capturing our playful imaginations, Media Lab.