http://www.earthhour.org/

More than 5,200 cities and towns in 135 countries worldwide switched off their lights for Earth Hour 2011, sending a powerful message for action on climate change. It also ushered in a new era with members going Beyond the Hour to commit to lasting action for the planet. Without a doubt, it’s shown how great things can be achieved when people come together for a common cause. 

Earth Hour Challenge
http://www.youtube.com/earthhour