If you are not ready to install a green roof yet, you can accomplish much the same thing by directing water from your roof into a rain barrel.

The main goal is to reduce the amount of storm water that runs off your property and into an overburdened sewer system. And if you save that runoff and use it to water your plants, lawn or garden, you will be both conserving a valuable resource and saving money on your utility bill.

Also see 30+ Ways to Save Money by Going Green