Arduino

According to the creators of the arduino it is "an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments", but if you ask me, it's simply "awesome". It's really great for screwing around with and also for making things that are somewhat useful. Some of the coolest are the UAV and the twitter reading robot, to name a few. For more info, check out their homepage.