I just bought an Arduino, which is a cheap open-source electronics board. You can program it from your computer and build all sort of interesting devices that can respond to their surroundings. It can take as inputs pretty much any kind of electronic sensor you can get your hands on (light, temperature, pressure, sound, force sensors, and countless others) and can use them to drive motors, switch things on and off, make music, run a web server, play pong, and so on. I just started tinkering around with it and it’s incredibly liberating to be able to get a computer to do something physical.
While looking around for interesting Arduino projects, I came across this incredible video via Make magazine. It’s an episode from a show hosted by Sylvia, a 9 year-old tinkerer and Arduino hacker.