Arduino: Open source hardware
Check out Arduino, a totally cool tiny embedded computer suitable for creating smart things - art, robotics, sensors and controllers. You program it on your laptop, and download the program to the device. I tried it, it was pretty easy. Had to touch wires and stuff though 
It appears to be a vibrant community with lots of people building compatible boards as well as add-ons which they seem to call “Shields”. For example here’s a shield to allow wireless communication between two Arduinos.
If you want to learn more, check out books called “Physical Computing” and “Making things Talk”.
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