What about radiant floor heat? My dad installed this in his garage when he built his new house couple three years ago. Basically you got a hot water heater, cement and and a bunch of hoses. Lay out all the hoses and zip tie them to the rebar, making sure they are even. Then run them all back to the hot water heater and a little circulating pump. (Theres a bit of plumbing involved with this) Then put some antifreeze type stuff in the hot water heater and start circulating. (bit more involved but thats the just of it) Fill with cement and your on your way to a nice warm floor and a toasty garage. Great thing is that the cement holds the heat, so you can open the garage door in frigid temps for like 10 minutes, close the door and the garage will be warm in a matter of minutes.
Only downfall with this is it has to be installed before cement is put into a garage. In your case, its too late, but if anybody is going to build a garage in the near future this is an excellent form of heat! Especially where it gets cold. It gets pretty extreme cold here and its a good system. Might be a little much then what anybody south of us would need though...
Other then that there are a few sources of heat. Propane, Natural gas, wood, oil, electric. I don't think it would take too much to get 220 put in your garage for electric. Then if you get ambitious you could install a wood furnace for when your working in the garage and then just have the electric running when your not in there.
