Anyone know of a water saving system for lawn irrigation?

Specifically, I am interested in maybe capturing rain water that runs off the roof gutters or just capturing rain water in general. It just seems to me the gutter is an easy way to capture a lot of water even with light rains. I guess the irrigation system would pull from the captured water, a tank, and then if it emptied switch to public.

I am wondering if systems like this already exist. If they don’t would it be difficult to rig? And, about how much do you think it would cost? Also, how many gallons would it have to be to be worth it? Does the water get filtered somehow? Can it be used in a pool also?

Another option would be capturing grey water from the house. If you have any links or information on that I would be interested in it as well. Especially prices.

To dig a well will be $4,000 and the area to be watered is not very large, so I don’t think it is worth it financially, but I do care about conserving water.