It seems a little crazy to pour drinking-quality water on the garden, on the car or down the toilet. Using rainwater where appropriate cuts the expense and energy needed to treat and transport water.
Install a rain barrel-a large container that collects water that falls on your roof direct from the drainpipes. Even a large barrel can fill in less than an hour in a heavy downpour. When you need to water the garden or clean the car, a tap at the base of the barrel allows you to fill a bucket or watering can.
From the Earth Handbook
Greening the Bend.