I mentioned to a friend the other day that I have a rain barrel in my garden, and she replied that she was going to buy one over the Big Home Improvement Store for around $75, because it was on sale. I told her that I had made mine for about $20 last summer. It sits beneath the downspout at the rear of the house, and one good rainstorm will fill up all fifty gallons. I have a hose coming out of the bottom so I can use it to water plants -- because it's sitting up on four concrete blocks, gravity does all the work for me (originally it was on logs, as shown in the photo, but we discovered blocks were way sturdier and less likely to tip over in soft dirt).
I should note that the pvc fittings at the bottom got knocked off when the husband ran into them with the lawnmower. You can see in the picture that there's a spigot handle which opens and closes the valve to the hose fitting. Anyway, after that fitting got knocked off, I replaced it with a simple hose attachment in the bottom of the barrel, so the hose is threaded directly into the cap at the bottom. When I'm not using the hose, I tuck the open end into the top of the barrel, in the hole where the downspout enters. This keeps it out of the way, and prevents water from leaking out (gravity at work, again).
I've got complete instructions on my Pagan website, over at http://paganwiccan.about.com/od/lammascrafts/ss/RainBarrel.htm, but honestly, it's pretty easy to build one. I think it took me an hour, tops. Anyway, don't go buying an expensive rain barrel. It's easy to make one -- I got my food-grade barrel from a guy selling them on Craigslist for $15, and spent about $5 on pvc pipe fittings.
I water my garden every day, and have yet to turn on the hose that's attached to the house this summer.