From the MAKE Flickr Pool -
Member Boss Sauce shares pics of his precision temperature controller using a PID mechanism.
A PID (proportional-integral-derivative) controller measures a specific variable (such as water temperature) in comparison to a predetermined setting (ideal temp). The controller then performs a corrective action via its control circuit in order to reach the desired setting (i.e. – increases power to the burner). Result = dinner cooked up just right.
PID temperature controller – Link
PID controller on Wikipedia – Link