I use a small (I think it's 20 watts) aquarium heater in the winter if I need it. I think it's best to put the fermenter in a water bath, and then heat (or cool, if it's summer) the water. That keeps the temperature even throughout and minimizes any risks.
I ferment most of my ales at 62 degrees (beer temperature, not ambient) but will go as high as 68 for some yeast strains. If your house is under 60 degrees, you may need to warm up the fermenter. But if your house is like many homes, room temperature is often too warm not too cool