A clever DIYer, Daniel Strohl, painted some aluminum cans black, drilled some holes in the cans, stacked them, and then put them in a box and, voila, air coming out of the box was fifteen degrees warmer than air going into the box. He then pumped the warm air directly into his garage. And with just a little bit more work, this exact same design could be a passive water heater.
We tend to think of solar power as a high investment advanced technology, but when it comes to heat, that's something the sun does awfully well all on its own. We should all do a TreeHugger search for "passive solar" every once in a while.