We love a good DIY project, and MaKey MaKey is a tool that promises to inspire several of them. It's already capable of creating a piano out of bananas, a Pacman controller from a paper drawing, and a keyboard out of alphabet soup.
It's essentially a board built on top of Arduino -- which is already capable of doing many cool things -- that connects to your computer via USB and helps you easily create a simple link between hardware and software. It's designed to allow the average person to create some simple things, but because it's based on Arduino, it's capable of a whole lot more if you're up to the task.
MaKey MaKey is currently a funded project on Kickstarter, and if you want to get one of the first kits, you can contribute $US35 to the project. That'll essentially preorder everything you'll need to get started when MaKey MaKey is ready to ship.