Makey Makey Go is a super-cheap invention kit. For $19, you get a USB stick and an alligator clip; use the two in tandem and you can turn (almost) anything into a keyboard or mouse button. Examples of potential uses include a Slip'N Slide that takes a photo as you zoom past, a donut spacebar, a dog bed that initiates a Skype call and a foil sword game that counts the number of times you hit an opponent. If you have an idea that requires more than one button, you just plug in another stick.
Source: Jay Silver (Kickstarter)