Tight on the good old green backs, and don’t want that window fan cranking away all night? The Thermo Cube plug should in theory pay for itself after a few hundred, perhaps few 1000 uses. Just plug in an appliance to the multprong AC adaptor, and when the mercury rises above 78 degrees its allows power to flow feely to the attached device. Drop below 70 degrees and it turns it off. Also, available in extra hot weather and cold versions. All models can operate loads up to 1800 Watts, 15 Amps, 120VAC.
Available here for $19.