Last year, WWF-Australia organized the inaugural Earth Hour. Creating buzz and turning heads around the globe, 2.3 million residents of Sydney simply turned off their lights for one hour. The result is immediate and gives a stunning visual of just how much energy is being used to power our cities. In that one event alone, Australia saw a 10% reduction in use of its power grid. In CO2 terms, that's the equivalent of taking 50,000 cars off the road for an hour.

EarthHour - Official Web Site
Information about Earth Hour - with video!