Not sure if this is a big savings or not, but a box fan used in a screened door or window opening in the rooms you are actually using during the springtime or early fall, rather than running the air-conditioning. Space heaters likewise in the winter months.
We have added turbines to our roof to get hot-air out of the attic in the summertime. They must be covered in the winter though.
edit to add:
How about those icepack neckties you can freeze/wet to keep cool while outside working or doing touristy things in the summer or while cruising the SavingAdvice.com forums?
We also use the settings in the freezer & refrigerator to keep the items just cold enough but not 'over' cold reducing the number of times the fan has to run.
You could also use a timer on a hot water heater if your shower/cleaning schedule was fairly consistent.
Any weatherization you could do to your home should generate some savings. Can't call it necessarily a gadget but how about those little foam whatchies you put inside electrical boxes to stop airloss? Or those cans of foam insulation you spray in cracks?
Getting some of that silvery tape stuff (I'm so technical

) that you use to seal up the duct work to your AC/heat unit. All those little leaks count!
I don't care what the experts say, I still like my solar tea jug which is in actuality an old glass pickle jar.
Consider the crockpot, microwave and electric skillet energy saving devices in the summer time as you don't heat up the house to cook.
A big one for us was doing the replacement windows and doors.
Hubster just added a solar powered motion sensor light outside. That way we don't keep the yardlight on all night. (Not that I ever did, I consider that a waste.) But accidentally, it would get left on upon occasion.
Here's a cool commercial on energy usage:
YOU HAVE THE POWER...SAVE ENERGY
relevant link:
93% less energy used in Zero Energy homes:
http://www.eere.energy.gov/buildings.../37547_zeh.pdf