Even more than your refrigerator, heating and cooling probably take the biggest chunk out of your home energy dollar. While better insulation is almost always worth the money, here’s a much cheaper idea: With a programmable thermostat, you can set your heater or air conditioner to take a break when you’re asleep or out of the house, and turn back on just before you get home. A bare-bones model costs about $25; you can install it yourself; and it could pay for itself in a single month. The new NEST programmable thermostat, designed by the original designer of the iPod, is a bit pricier. But you can control it from a smart phone, and it even programs itself.