There was so much response to the posting about rain barrels in Garden Rant that I want to pass on a link to Rebecca Chesin at her website with lots of information about making a rain barrel, and information from various dependable sources about the safety of water that has washed off the roof. I never thought that asphalt shingles or that the chemicals used in wooden shingles to make them rot-proof or moss resistent might make the water unsafe for use in the vegetable garden. The moral of this lesson is Know Your Roofing Material.
You might want to also check out the patent pending rain barrels at http://www.aquabarrel.com. They also have downspout diverters and downspout filters too.
Thank you for mentioning aquabarrel. I especially liked the large diameter overflow. The rainbarrel discussion also taught me that wholesale food products are often shipped in plastic barrels. Who knew?