Back in science class we (should have) learned that white surfaces reflect all visible light. For centuries, people living in sunny climes have taken advantage of that property to reduce the heat load on their property, by creating white roofs. White roofs reflect the sun's rays and can make a considerable difference in one's air-conditioning bill.
The state of California mandates white roofs on new commercial buildings to decrease the load on the electric grid and commensurate carbon dioxide emissions. The same benefit can be realized by homeowners in the sun belt. A recent study found that a 1,000 square foot home with a white roof could save ten metric tons of CO2. This could translate into a 10-15% drop in your air conditioning bill.
New regulations soon to take effect in California mandate energy-efficient covering on sloping roofs, too, including residential housing. If you're not a fan of white sloping roofs, though, don't panic. Since half of the heat in the sun's rays comes from the near-infrared, non-visible part of the spectrum, new roofing materials in traditional colors are available that still reflect these rays and meet the state's requirements.
I doubt that the savings will cover the cost of treating your existing roof material, but if you're in need of a new one, consider going white with a flat roof, or using heat-reflective materials on sloped ones. The cost of electricity is headed nowhere but up.