Rain barrels are a cheap way to capture rainwater from downspouts to use for later irrigation or watering. One quarter inch of rain on a 12'X12' square area is 5 gallons of water. It doesn't take much to figure that the roof on the average house with 1/4" of rain will see 100 gallons or more depending on the roof size. This captured water saves in water and sewer cost.
There are many sources online and in many localities selling rain barrels, gutter screens, etc. to help insure good clean water.