Before I moved my fermentations to the basement, I used a "BrewBelt" on my plastic buckets. I do not know how well that would work on glass.
Some people I know take an incandecent light bulb, and just cycle that on and off to produce heat. If you are using glass, however, I would recommend some paper or cardboard around the carboy. I do not recall if incandecent light will turn beer skunky or not, but my suspicion is that it will.
I have also heard people have good luck with an electric heating pad... not up against the carboy, but in the fridge. They also make a "Fermwrap" I think is the name... that works like the brewbelt, but the heat is more evenly distributed.