With full-size refrigerators starting at $400 and passing $4,000 for some high-end, side-by-side models, you may be tempted to hold onto your old fridge for as long as it keeps humming to save money. But according to Energystar.gov, refrigerators made before 1993 cost an extra $50 per year to operate compared to new Energy Star-qualified models. A box made before 1980 costs about $150 more per year to run.

The website also points to a potential 20 percent savings from installing more efficient heating, ventilating and air-conditioning, or HVAC, units in your home. But before you replace your HVAC system, check for air leaks, which are likely culprits for your energy dollars disappearing.

Through 2010, you can get federal tax credits worth 30 percent of the cost, up to $1,500, to install Energy Star appliances, windows, insulation, roofs, HVAC systems and water heaters in an existing home that is your primary residence. A number of states also offer rebates for purchasing similar energy-efficient upgrades.