If you have room in your garden, and your local authority allows, you could probably get a tank from a rural supplies company to install.

If that looks too ambitious, you can get gadgets that you insert in a downspout to divert the water into a container and then reopen when the container is full. If you're a home handyperson you should have no trouble.

If you have a free-standing shed/barn you could capture the water from that roof. Do ensure you have a spillway for the overflow - an ephemeral streambed, perhaps... leading into a dunk pond for watering seed trays and such.

It is possible to be very elaborate, or keep it as simple as a series of buckets under a leaky gutter. Mine's not quite that primitive: I have two 44 gallon plastic drums on a tank stand. One has a spigot so I can draw off, and the overflow goes into a soakpit beside a thirsty elderberry. I also have an old concrete tub that holds about 30 gallons, lined with a butyl rubber sheet, that catches water from the woodshed roof. I mainly use them for pot plant care to ensure they're not forced to drink the chemical cocktail I get through the tap. (top grade drinking water, but it tastes Vile.)

Grey water: about the nearest I get is capturing the rinse water from the washing machine in summer for distribution to deserving shrubs. I think it does depend on your soil type and whether or not you are likely to get leaching into any artesian system. If you wanted to direct the flow from your kitchen, laundry, bathroom onto areas of your garden it may pay to put it through a filter system (bark or shingle) or some of the grass species that take up concentrations of various chemicals.

If you have a composting loo, distribution in a woodlot is the traditional answer. Just be prepared for lots of wilding tomatoes!

A rain barrel can be as simple as putting a large garbage container under your downspouts. (You will need to cut the downspouts to length and also cover the can with screen--I use bungee cords to keep it in place--to keep mosquitos out and animals from downing.) You can then dunk in a watering can and away you go.

It also isn't hard to add a spout to a barrel. I got free 55-gallon barrels from our local water treatment plant to use for the purpose. I attach a soaker house to one of my barrels and a regular hose to the other.

I use grey water simply by leaving a bucket in my bathtub and putting it under the faucet while waiting for the water to warm for my shower.

Monica

Here is a link that might be useful: Making a Rain Barrel

We sat up rain barrels just this month. Our diverters came from Real Goods but looking at the simple design, we probably could have applied our brains and saved some $$. The diverter is basically galv. sleeve that fits onto the down spout. The diverters saving grace is that the 'front' face pulls forward and down to make a small transport trough to the barrel. When barrel is full, push the face back up into place and the water continues down the down spout.
Our barrels are metal 50 gal 'food grade' barrels (previously held pie filling!) that came with tops and locking rings. They were cheap, clean and available y2k barrels but I think plastic would have been easier (thinking of drilling spicket holes). A friend up north has a source (state pen) that she regularly picks up barrels and 5 gal food grade buckets.
We haven't figured out an inexpensive plan of action to divert our grey water because our house sits on a cement slab and our laundry room is in the middle of the house :(

I set up a rain barrel off of our garden shed this past Aug. I bought the barrel at a garage sale (it's white plastic; thick and kind of translucent) and guttered our shed on one side. My total bill was about $50, and I'm really happy with my set up. It doesn't take much of a rain to fill the bucket, and because I have the barrel sitting on top of two cinder blocks, I have gravity on my side, so there's decent water pressure.

Heres is a warning about grey water. DO NOT STORE. The soap and other organics (food stuff from the kitchen sink)will very quickly spoil if stored. It is much better to run grey water straight out to plants in the garden. Do this underground if you can. The bugs will take care of the organics and the plants will get a good watering. You can set up a system to diverate the grey water to various plants by running the house pipe to a distribution point. This is all underground. The distribution point is simply a spot where the grey water can collect and can be as simple as an overturned large plastic barrel. From this point you run a few pipes leading to shallow ditchs leading to plants. The shallow ditches are filled in with sand or small stones and covered over with mulch. All this has to be at an incline directed towards the plants to be watered. Sounds like a long of work but it really is simple. Oh and use the smallest pipe you can. This creates more flow (pressure is the wrong word) but think of it this way; a smaller pipe will have more water filling it than a larger pipe would for the same volume of water. The more the pipe is filled up with water the easier it is to push along any particles that could clog up the pipe.
The other option is to run the grey water to a small (or large) bog area/pond. There you can enjoy bog plants and they will take care of the organics.
BUT DO NOT STORE GREY WATER! Use it as you make it.

Now run off from the roof is another matter all together. You can safely store this water. You might want to invest in a diverter on your gutter downspout that is "open" during the first few gallons of rain coming down the drain. This allows any leaves and trash to be cleaned out of the drain (goes outside). When the diverter is "off" clean water fills your storage containers. This water came be stored for a fair amount of time. Do remember any water stored over a long period of time will spoil.
Hope this helps :)
Mike

It started out as a simple experiment - this past summer, we were able to water almost all of our container grown plants using grey water, approx 20 pots of varying sizes, including 3 large ones which held 2 young raspberry bushes each. We kept a pail and a small watering can by the kitchen sink and collected most of the non soapy water in it, when it was more than half full my kids,DH or myself would run it outside to water something. This happened about 3 or 4 times a day. Of course it was easier because the kids were off for the summer and my younger daughter seemed to enjoy doing it.
Compared to last couple of years where we would water each pot by hose every evening when it hadn't rained, we felt or at least I did, it wasn't much of an effort at all, we got to stop and smell the flowers every time we watered and it definitely saved a lot of municipal water.
We also installed 3 rain barrels this summer, under down spouts to collect rainwater. Our water bill in July (rainy month) and Aug was comparable to the winter months, last years summer bill was 2.5 times that of the winter months, so we feel purchasing the rain barrels and collecting the grey water has paid off. Hope this helps.
Mary Anne