If your house has central air conditioning, the air conditioner will probably be the biggest user by far. Although used only a few months of the year, the annual cost can be much greater than the annual cost of your refrigerator, which is typically the next largest user. In hot climates, the annual air conditioner cost can exceed a thousand dollars. You can get a very rough idea of what your air conditioner is costing you by subtracting the electric portion of your bill in a spring month when you aren't using your air conditioner from the electric portion of the bill in the summer when you do use it. This gives you the monthly cost. Multiply this by the number of months you use your air conditioner to arrive at your approximate annual cost.
Refrigerators are typically the largest users in houses without air conditioning or in climates where the air conditioners are used only a few days of the month during the cooling season. If your refrigerator is more than ten years old should consider replacing it. New efficiency standards went into effect in 1992, and older refrigerators are typically two to three times more expensive to run than a new unit. For more information go directly to the American Council for an Energy-Efficient Economy's list of most efficient refrigerator-freezers.