In our house we have a very simple system to capture rainwater into a tank. We use it to water our vegetable garden during the dry season.
We use a 1100 liter water tank. I would have liked something bigger, but there just wasn't enough space for it.
1 is the exit from the roof; water from various roofs in the house gets directed here.
2 is a T connector. On the horizontal side it goes to 3, where it enters the water tank. On the vertical exit side it goes down a relatively long pipe, all the way down to 6, which is a valve or faucet. That long pipe is the "dirty water collection pipe". When it starts raining, the first charge charge of water can be quite dirty from all the dirt on the roof. This dirty water fills the long pipe. The dirt accumulates in the bottom of the pipe, and finally cleaner water will have a chance of filling the tank when the water level in the pipe reaches the T connector.
4 is the overflow pipe. When the tank gets full, any overflowing water will exit through here.
5 is the water tank's exit pipe.
6 is the valve controlling the output from the dirty water collection pipe, as mentioned above.
7 is a normal faucet coming from the water tank. This is where you would plug a garden hose.
The trick is to adjust 6, the valve of faucet at the bottom of the dirty water collection pipe, so that water and dirt can trickle out of it. That is, don't leave it fully closed. The pipe will fill up shortly after it starts raining, allowing the rest of the rain to reach the tank; finally the pipe will empty itself for the next rain.
Does it work!
It works very well! We use the stored rainwater for plants in the vegetable garden. We used that water for construction of the garden's planting beds, to make mortar for bricks. Although we have managed to empty the tank a couple of times during very heavy use, normally it rains often enough that the tank stays almost full.
Note that the tank is basically one storey above ground level. This gives barely enough water pressure for a slow-running garden hose.
2022 Update: Since the installation of the original system in 2015, we have added a flow-activated pressurizer pump so we can irrigate comforatbly.
T.H. Thomas and D.B. Martinson, Roofwater Harvesting: A Handbook for Practitioners (PDF)