I've just bought and installed one of these
It's brilliant, you set the target temperature and it will either switch on the fridge or a heating device depending on where the current temperature is. You also set the tolerance, to a minimum of 0.5C. i.e. If the target temp is 20C, the current temp is 16C and the tolerance is 0.5C the heater will operate until 20C is reached, then switch off. It will switch on again if it drops to 19.5C. The fridge will switch on and off in the same way.
Means of course that the seasons now play no part in brewing!!
I bought the fridge at an auction for $10. The TempMate itself cost about $110 including shipping and the project box to house it for $20. I'm using a heater pad which was $60.
The wiring is relatively easy, but you need to decide whether to do it yourself or get a sparky. Googling TempMate will take you to a thread on AussieHomeBrewer which has wiring diagrams.
I've wrapped the fermenter in an insulation layer (a camp roll) so that I can jam the probe between the two to closely record the actual beer temperature, rather than the air temperature.
By the way the jiffy box that CraftBrewer sell to house this isn't big enough.
Highly recommended