I like to collect rainwater, and it's free. However, I prefer collecting it only on days when it rains heavy and for quite a while. That way, it has "flushed" away any pollution, etc. from the air and off of my roof. So that pretty much any rainfall after that is nice and clean. I collect it in a big bucket on my back porch and then bottle it up into a bunch of one gallon plastic bottles. I keep plenty of filled bottles in reserve, so that I never run out. But I rarely even use half of what I keep in stock. It's mainly for Summertime, when the days of rain are fewer and farther between. I also rotate through it, using the older bottles up first, so that the rainwater stays nice and fresh and doesn't just sit around not getting used. Oh, and I also bottle it up asap after it rains, so that it hasn't been sitting around outside for too long either, having leaves and other debris falling into it. Now that I've been collecting my own rainwater for a while, I've gotten the hang of it. It only takes a few minutes out of my day to restock my supply, and only when I've used up about half of it.
One funny thing that I've encountered was that my neighbors seen me bottling it up once before and were feeling sorry for me, lol. They thought I had no running water and was using it for drinking. But, I cleared up that misconception and we all just laughed about it in the end.