No, there is not an ENERGY STAR label for attic fans because they use more energy than alternative attic cooling methods such as passive ventilation. Attic ventilation is important to keep roof shingles from degrading due to extremely high roof deck temperatures and to allow moisture that might get trapped in the attic to vent to the outside. Attic fans mount in your attic in a gable vent or through the roof deck to mechanically vent out hot attic air in the summer.
Passive Ventilation is another method for attic cooling which uses soffit vents and gable or ridge vents to allow the natural flow of rising hot air to reduce the attic temperature. Mechanical attic fans can suck cool, conditioned air from the house making the air conditioner work harder and using more energy.
In addition to passive ventilation, to reduce your energy costs, air seal your attic and add insulation (cellulose or fiberglass) to your attic to at least R-38. That will create a barrier that resists the heat from the attic radiating down into the house. Our Home Sealing Do-it-yourself Guide can help you seal and insulate your attic (http://www.energystar.gov/homesealing).
Note that attic fans are sometimes confused with "whole-house" fans, which are mounted in the attic floor, and pull cooler air from outside into the house to lower the temperature. Whole-house fans can be a good alternative to air conditioning under certain conditions.