Superb Idea: The Rainwater Pillow
The Rainwater Pillow system consists of an intake filter connected to a flexible water storage bladder, and an outflow hose powered by a small integrated pump. The standard 1,000-gallon Rainwater Pillow measures 9' x 11' x 2.5', and it can be positioned in any convenient spot (such as underneath a deck). ... According to the manufacturer, a roof measuring 40' x 40' would provide enough runoff during one inch of rainfall to fill the 1,000-gallon Pillow.
So this system is better than a rain barrel if you want to be able to collect large volumes of rainwater for use in your yard or garden. It would work especially well, I'd imagine, in single family homes. It has enough water pressure for yard irrigation, has overflow valves, and features a remote control for the pump if you want to store the pillow itself someplace out of the way.
At $2,200 it's pretty expensive, but in Los Angeles a family might spend upwards of $100 each month on outdoor water use. At that rate, this system could actually pay itself off. And, of course, it helps prevent polluted runoff from flowing into storm drains.