We bought a 1200 gal galvanized metal cistern for $200 from a farmer's widow who wasn't using it anymore. It's the kind that is shaped like a cylinder with a pointed top. I still see them on farms in this area although most aren't being used as cisterns. That's too bad as rainwater is some of the purest water you can have and all plants love it. Our house is quite small; about 1100 sq. ft. Because of the way the gutters are configured, we can only catch rain from about 1/2 the roof. It is truly unbelievable the amount of water you can catch, even from a small roof footprint like ours. We've had about 6 inches of rain the past few weeks and the cistern is full to overflowing. There is an opening in the bottom of the cistern where we've attached a faucet and tubing. We will buy a small pump to use when the vegetable garden is planted to put water through the soaker hoses. We also have a roof washer setup where the first flush of water off the roof bypasses the cistern. You need this to keep all the debris, bird droppings, and other material from entering your cistern. It is a pretty simple setup, but necessary, especially if you have a composition roof. This works so well we are thinking of getting a 2,000 gal. food-safe cistern to catch water from the other half of the house, just in case we need access to potable water (would still need some treatment before drinking, though). There are homeowners living in the hill country west of Austin, TX that supply all their housewater from cisterns because no municipal water is available. If DH and I build a new house, definitely a rainwater collection system would be part of the plan. In central Texas where I live, you can get 10 inches or more rain in the space of a few weeks (or days:)), then nothing for months. Cisterns, which are basically a residential reservoir, help ease that imbalance. Plus, the water is so much purer(unless you are downwind from an industrial source of air pollution). It's generally pH neutral soft water without all the chlorine and other by-products inherent in municipal water supplies.

Here is a link that might be useful: City of Austin Green Sourcebook

I read some similar comments about the laws in Colorado on a garden web forum. Several people were confused but I think the general concensus of it was that it is not illegal to collect water from your roof, but it is illegal to sell that water.

Here in drought-stricken Australia this has gone from being illegal in the suburbs to being encouraged to the point where the government gives you a rebate if you hook your system up to at least flush the toilet. Drinking water of course gets a little more complicated. You need to check whether all the surfaces the water will be flowing over are suitable, eg, don't contain lead (one of our government departments decided to make the most of their large roof and forgot about that little detail). Also you need something to keep mosquitoes out.

We just completed installing individual containers at our downspouts that will hold over 600 gallons (with the possibility of adding more). I am located in Kansas and our town has a water shortage and restricted water use.

Check with your local city/county government about the type of water containment you can use. We can use above-ground containment ONLY.

We got our containers at a local farm supply store where they had a large selection of types, sizes, and styles. They had the traditional-looking plastic 50-gallon barrels (already with a spigot on the bottom) and 55-gallon containers without a spigot (on sale for $29.99 each) and we also got 2 plastic containers (165-gal. and 215-gal.), and had to fit them with spigots. Luckily we were able to find plastic fittings in the plumbing department of the local hardware store that we were able to screw into the base bung hole. So think your possibilities through - beyond the traditional-looking barrel. You also need to elevate the containers (concrete blocks) since they are gravety fed, or you can install pumps to move the water and provide more pressure for regular hoses.

We purchased one container that was a low-profile, mushroom-shaped (oval) container and we painted it to look like a giant lady bug.

The downspout diverter can be any number of things (as you will find if you Google the subject), but the one we choose was from Gardener's Supply Company (item 33-991 - link below). Once our barrels are full, the dirverter we chose automatically sends the overflow down your existing downspout, and was easy to install using a hacksaw. Other types require you drill a hole in the barrel and install a short length of garden hose for the extra water to exit the full barrel.

Also from the farm supply store, I purchased "Mosquito Dunks" and placed one in each barrel. They look like a doughnut and are designed to place in standing water to kill mosquito larvae (for 30 days).

We have created landscaping and a small garden (22'x6') (new house and yard) that are watered with solar irrigation drip kits (also available from Gardener's Supply Company - item 36-517). I fill a large decorative container with water from our containment barrels, drop in the pump, and place the solar panel in full sunlight (which we always have an abundance of in Kansas), and it feeds up to 20 drippers.

-Grainlady

Here is a link that might be useful: Gardener's Supply company

We have a lot of folks in Hawaii (on the Big Island, the Island named 'Hawaii') who use rainwater catchment as their only source of household water. The County does have roadside water spigots where some of these folks get their drinking water as well as can get water during times of no rain. The Hawaii College of Tropical Agriculture and Human Resources has written a booklet about the whole thing. Here's an excerpt from their webpage about the book along with a URL to get to it:

An estimated 30,000 to 60,000 people in Hawaii are dependent on rainwater catchment systems for their water needs. Despite the fact that so many people own and use these systems, very little information has been available about their design and maintenance until now. This publication addresses water quality issues from the raindrop to the faucet. It includes chapters on water collection,water storage,water treatment, water testing, and firefighting concerns, and it gives an overview of the typical kinds of catchment equipment used in Hawaii. These guidelines are intended as a practical reference on the major concerns and best management practices for proper maintenance of rainwater catchment and storage systems.

Guidelines on Rainwater Catchment Systems for Hawaii
52 pages, color cover.
ISBN 1-929325-11-8
by Patricia S. H. Macomber, Department of Natural Resources and Environmental Management,
College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa.

Here is a link that might be useful: CTAHR water catchment book

For a very cheap solution, stop by your local soda bottler and ask if they have any syrup drums. The coke bottler near my house gave me as many as I wanted for free (they use them as trashcans at the plant). These are food safe and similar to the 55 gal drums that are sold at garden stores.

To turn these into a rain barrel simply cut a hole in the top to collect the rain from the roof (or downspout) and drill a hole in the side to attach a hose spigot. The total cost of the building was about $5.

As mentioned, these are gravity fed so you may need a way to increase the pressure. I built a stand out of some scrap PT 4x4s and fence pickets that gives me enough pressure for watering the garden.

We've been relying on rainwater for several years now and have quite a few firends who do the same. A few years back a Park Service employee offered to test our water because he was curious. The next time I saw him his jaw dropped. He said he'd done $800. or $900. worth of test and found... nothing. He couldn't believe it. Another set of our friends had their water tested and got the same result.
OK. "Nothing" isn't quite accurate. Both samples had a higher level of dissolved solids but nothing dangerous.
Of our various friend who collect rain water one set does nothing before drinking it, another set adds chlorine, then filters it, another set boils it in a solar oven, and we run ours through 2 sediment filters then add chlorine and Britta filter it. So far so good.
I'm a little nervous about water-born diseases though. We're trying to figure out something a little more reliable for the future. I just hope we don't get sick first!