One oft-mentioned way to "go green" is to buy a rainbarrel to collect water from your roof. There are a few things you should know. I write this as an advocate for capturing and utilizing the runoff from roofs, driveways and uphill neighbors' yards. There's a role for rainbarrels, but it's often overstated.
First, a bit of calculation shows that a 2000 square foot roof captures 1250 gallons of water from a 1 inch rain. Divide that by four downspouts, and you have about 300 gallons coming into the rainbarrel in a modest rain. If an average rainbarrel holds 50-75 gallons, most of the water will not be captured. The percentage caught will be even lower if the rainbarrel still has water from the previous rain--likely, given that people tend to forget to use the rainbarrel water in their gardens.
So the question becomes, is the rainbarrel really making a difference, or is it just adding to the amount of "stuff" in the world? I have a rainbarrel--rescued from a dumpster and retrofitted with a screen to filter the incoming water and keep out mosquitoes--but the water doesn't get used much, so I just leave it on slow drip, to let the water empty out before the next rain. Attaching a soaker hose is another way to slowly release the water into the garden.
Unless you're really going to use the water, it can be more aesthetic and consequential to direct the roof water into swales or concave raingardens that can accommodate the hundreds of gallons generated. Or, if you're serious about catching and using water, check out this system at the West Windsor Municipal Center. It looks like it holds a couple thousand gallons.
The giant rainbarrel (you can get these at farm supply stores) is raised up to add pressure to the gravity feed over to what looks like a community garden.
It's fun to see a 1970s counter-cultural dream come to life on municipal land in the 21st century. The central concept was easy living by working with nature. We'd store the rain from the sky, capture nitrogen from the air by planting legumes, use mulch to discourage the weeds; and interplant to make full use of the sun's gift of energy; then sit back and watch the garden grow.
Implementation was not always perfect. In Durham, NC in the 1990s, realizing that one small rainbarrel wasn't catching much runoff, I picked up a dozen 50 gallon drums from the local Coca Cola plant, washed out the dregs of syrup, then drilled holes to connect them in series. They weren't very aesthetic, and the 220 gallon plastic cisterns I bought at a farm supply store to replace them with didn't exactly blend into the landscape either.
These experiences led to dreams of bladder-like rainbarrels that would deflate when empty--far more efficient to transport from factory to home. After a big rain, they'd bloat like a Michelin Man in a tire commercial, but would shrink down behind the shrubbery at the base of the house's foundation inbetween rains. That was the dream, and it's nice to see that these now exist, under the name "rain bladder". Though they are pricey, that could change if they became standard installations on homes.
Another alternative to the standard rainbarrel is what I call a "fillable, spillable minipond", basically a black tub placed under a downspout, which in turn overflows into the yard. It provides open water that's nice to dip your hands into while working in the garden, and a blank canvas for nature's crystalline creativity in the winter. The tippable aspect proves handy if mosquito wigglers appear.
After all the experimentation, the cheapest and most aesthetic approach to capturing and slowing down runoff from our home and driveway turned out to be the yard itself. Some downspouts send water flowing across the lawn, or into a shrub boarder, or into concave raingardens that accumulate a few inches of rain that then seeps into the ground over a day or two. (We have miniponds, too, dug deeper for extra capacity, but the attractive open water requires special attention to discourage mosquitoes.) With the help of some low berms and hollowing out here and there, the yard itself becomes the bladder, and the ground, rather than the rainbarrel, becomes the reservoir that deep-rooted plants and trees can draw on during droughts.
The little rainbarrels can still play a role, but compared to all the water a roof generates, and the potential holding capacity of a yard, they really are a drop in the bucket.