This one was designed to control air conditioner which seems lacks some automation. Anyway this project suits for many purposes and seems to be very simple: Arduino, thermal resistor and servo motor that turns conditioner knob.
The project has some shortcomings like draining 9V battery with 5V voltage regulator. And probably code needs to be tweaked for more efficiency. Anyway this a good start and great example of simple solutions that may help to lower electrical bills.