Number 21 on TIME Magazine's list of 51 Things We Can Do to fight global warming now is one of the simplest: wrap your water heater with a thermal blanket.
The cost? $10-20. The benefit? Your water heater will lose less heat over time, lowering your bill for electricity or gas (depending on your type of water heater), lowering your bills and your household carbon emissions (approximately 250 lbs. of CO2 per year).
For the price, I plan to do this for my apartment. There's not much I can think of that will have this much effect for such a small, one-time price.