What is the device that has an exterior thermometer probe that attaches to a home furnace to save energy?

I recall seeing one on Ask This Old House a while back. It was a device that had on probe that led outside that used the reading from it to determine how to run the furnace more efficiently when used in sync with the home thermostat. I want to get one as it seemed to have been a good way to save energy, but, it's name escapes me now.