This December, Wal-Mart hopes to sell its 100 millionth compact fluorescent lightbulb (CFL). It costs three to five times as much as a conventional bulb, but it burns 75% less energy and lasts years longer. Wal-Mart is one of the backers of 18Seconds.org, which offers fun facts like this: if every American bought one CFL, the effect on greenhouse gases would be equivalent to taking 2 million cars off the road. Not a big fan of the swirl? Experts say even better technology—leds, or light-emitting diodes—is the next big "green" light.