The Bodybugg is like having a personal trainer strapped to your triceps. The 2.9-ounce gadget uses four sensors to calculate your energy expenditure with 92 percent accuracy&8212;on par with in-the-lab measurements. An accelerometer tracks your movement; two thermometers gauge body
temperature (one's for your core, the other for skin); and a
galvanic-skin-response sensor measures sweat. Upload the data to bodybugg.com, and the site will chart your daily calorie burn against your intake. $500
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.