Since there's a very good thread on swimming pool water, I thought I'd troll for other sources that people don't think about. Yes, the toilet tank and hot water heater have been discussed, but there's another. If, like me, you have kids, and have a portable basketball hoop on the edge of the driveway, don't fill the base with sand, fill it with water! 20 gallons right there. Filling it 3/4 will prevent it from cracking in the winter.