I've been using several of those plastic water "carboys" for storing water since I started doing that last year. I think I have 3 or 4 of the 5 gallon ones and a couple of the 4 gallon ones, one of which I've never opened. I originally got them to use as secondary fermentors for my wine making efforts, but I realized that I had more of them than I needed for making wine, so I filled them with tap water and put them in a corner of the basement.
I really should invest in a good water filter to filter rain water if I needed to do that. There's a lot of water sources here where I live, but they all would need filtering to make them potable.