You guys are making me love the simple woodstove in my garage. It's especially nice as a place to set tools you're using, so they are warm when you pick them up!
It came from a neighbor's equestrian arena, where they had it in a small side room, about 12 by 20' and it heated the room too much. In my 24x30 garage, it's perfect! Fuel costs are basically nothing. I feed it with junk mail, and I'm still burning the pile of off-cuts and pallet material left over from building our arena two years ago.
The best thing I ever did for my garage was to install a passive solar heater. It's just a clear plastic panel that sits in one of the garage door openings. If you open the door six inches, the sun shines on the dark metal door, and the heat rises through the opening in the top, cool air comes in the opening at the bottom. At night, simply close the door, and the heat stays inside. If you're working in the garage during the day, you can open the door all the way, and let the sun shine in onto the concrete (thermal mass) and you get the added benefit of beautiful daylight inside the garage. I can't count the days I've worked on some project right inside that clear panel, with the warm sun shining on me, in great light, and a winter wonderland outside that plastic.
The best part is that the whole thing cost only about $100 to build. It's portable, so in summer I move it around to the side of the garage, where it hangs out of the way, so then I can use both garage doors. If I only had one door, I'd have to hinge the solar panel, so I could open it to get in and out. As it is, I use the area in front of the "solar door" to park bikes, or for projects that don't need to go in and out often. I generally leave the solar panel in place all winter.
Another smaller passive solar air heater hangs on the south garage wall next to the door, and serves the same purpose.
Here are some pics and diagrams: