Rainwater is a great resource, no chlorine or fluoride to go into your plants.
Apparently 1" of rain on a 1,000 s.f. roof can produce 6,000 gallons of rainwater. That's a lot of water! Why not use some of it to water plants instead of tap water? Until this one comes to the US look for local options for rainbarrels. The hardware is easy to find at your local hardware store, the barrel might be a little more difficult to obtain.
photo courtesy of: http://www.basvanderveer.nl/index.php?id=2