The evidence is pretty irrefutable: Most of the energy you use in the laundry goes toward heating the wash water. Switching to cold water washing is the single greatest way to save energy in the laundry room. It can be a big adjustment, it's true, but there are ways to make the switch and still get clean clothes: Consumer Reports gave one cold-water washing formulation, Tide Coldwater, high marks based on recent tests. There are also lots more cold water cleaning options on the market today than there were even a couple of years ago. Washing machine manufacturers are also tweaking their products to make them work better with cold water detergents.
You don't always need a special detergent to clean clothes in cold water, either. Your regular hot water detergent will work fine for some cold water washing chores, especially if the items involved aren't white and don't sport greasy stains. Experiment with cold water washing, at least once or twice. The energy you save will be your own.