Check out this guy's DIY project for creating a passive heat solution to heat his garage. It's so simple and elegant, it pretty much qualifies as genius. Basically it boils down to:
1. drink a lot of soda pop to get empty cans 2. build a big plywood box 3. paint the cans black, stick them in the box, and caulk the heck out of everything 4. put some plexiglass over it and point it at the sun 5. vent the air from it into the garage.
The best part? All of these materials cost him less than $50, and he's getting a fifteen degree temperature differential.