I am using the Williams Brewing Heating Pad made for fermentation chambers. It is pretty nice, has adhesive on it to attach to whatever surface you want, and heats well. I have it hooked up in my ferm fridge on a two stage controller, when I first plugged it in the fridge was at 36 and it took it up to 68 in less than an hour, with a good amount of liquid in the fridge.
Primary: White Rose Belgian Wit
Carbing: Special Bitter
On tap: Gimme S'more Stout, Tweaked IPA, Hoppy Sunday Barleywine, Midnight Tryst Schwarzbier, BBBBB (Big Batch Bourbon Barrel Belgian)
Bottled: Rahr Whiskey Warmer Clone