Well. It's hardly free though. Every 10-15 years or so, something like a pump will go out requiring a large bill. Sometimes a new well has to be dug, that's a huge expense we luckily have not had yet, but some of our neighbors have.
We have well water and it is far from free. We have to maintain the well, pump and drain fields. In addition to that, we have a water softener, UV light and filters. The softener also requires salt each month.