You can always place a rain barrel under a downspout to collect rainwater from your roof and feed it to your lawn or garden. This is a good practice. Though the trouble with many of these systems is low water pressure from the barrel. If space is a consideration, consider a Rainwater Hog, with units that can be screwed together to provide storage in 50 gallon increments (shown). Some newly constructed green homes place cisterns underground, which can be monitored via sensors and automated to irrigate your lawn or gardens. Sweet, though such systems will cost you thousands.