Heat Pump
An air-sourced heat pump is basically an air conditioner that can be run in reverse in the winter (taking heat from the great outdoors and bringing warmth to your home). As with air conditioners, they have a problem spanning too much of a temperature differential -- the carnot efficiency drops as the delta goes up.
Geothermal heatpumps don't have that problem (since the underground temps are more stable) but cost a lot more to put in, and aren't always practical, as they require long ground loops trenches to be dug into your yard. The payback timeline is 3-5 years.