Props go to m2 for this: so if you buy a "Mini Digital Temperature Controller Thermostat Aquarium" off Ebay ($24 shipped) you can make a cheap temperature controller (see attached image).
It will in fact control the power to one outlet for heat & one for cooling. So you can use it to cycle a pump or heating element for mashing, and control a fridge for constant temps during fermentation, without much trouble.
One limitation: it reads only in degrees C.
Compare to $60-75 for a new Ronco or whatever.