Turn your lights off this Saturday from 8:30-9:30 PM to take part in Earth Hour. "Earth Hour, the public awareness raising campaign on climate change is showing signs of being the greatest voluntary action the world has ever witnessed." - http://www.earthhour.org/home/
This event really seems to be gaining some momentum in its second year. Check out their site for ideas, suggestions, and to see who is participating - you might be surprised.