I don't know how many cities are participating, but Toronto is attempting to "turn off the lights" from 8 pm to 9 pm tonight for "Earth Hour". Sydney, Australia started this program and even though it was the first year had a decrease in energy consumption of 10 percent, equivalent of taking 48,000 cars off the road for an hour. It is right in the middle of a Toronto/Montreal hockey game so my husband is a little grumbly about it but he's on board. We are breaking out the candles and a board game!
Earth Hour US - Earth Hour 2008
The Associated Press: Sydney Goes Dark for Earth Hour