Our garage has in-floor heating using a propane boiler. It is a weekend only place for us and while we keep the temp at 40 F, it is still better than ambient plus it lets us keep the water running all winter.
With propane approaching $5/gal these days, however, it is getting hard to justify the cost for a couple of weekends a month. Next winter I am thinking of blowing down the water lines and making sure the heating system is adequately freeze protected and just letting it freeze.
The other thing I am considering is installing the old Earthstove we pulled out of the cabin this winter when we installed a Blaze King. I know that installing wood stoves in garages is not a generally accepted practice but I know that it is done. We do not store fuel inside but do park the 4 wheelers, snow machines and the old MG inside in the winter. I am not sure if this isn't too much different from our garage at home - both the natural gas fire burner and the water heater are in the garage.
My wife thinks this isn't a good idea but I think it is manageable - what do you guys think?