If you have an attic, you know just how hot it can get in the summer — temperatures in excess of 150°F are fairly common. (It can get even worse if your roof has dark-colored shingles.) Having a massive volume of air at 150°F just above your living spaces can seriously increase your cooling bills in the summer time. There are several steps you can take you reduce your cooling bills — one of the most cost-effective is to add thermostat-controlled fans to remove that hot air from your attic. 
We keep the thermostats on our attic fans set at 100°F to keep temperatures down. The electricity we use to keep the fans running is more than offset by the electricity we don't use to run the air conditioner. When selecting attics fans, always consider the volume of your attic. The general goal is to have all of the air in your attic replaced every minute until the attic temperature approachs the temperature of the outside air. During the day, when the sun is shining, there is so much energy hitting your roof — especially if it is dark-colored — that your attic fans will run all day long without a pause. An average attic fan for sale at Lowe's will suck out about 1,500 cubic feet per minute, which may sound like a lot but really isn't all that much. (We have four 1,500-cubic-foot-per-minute attic fans in our attic, which sounds like the house is about to take flight when all four turn on at once.)
A solar-powered attic fan may seem like a great idea because the fan runs essentially for free. The problem, however, is that the typical solar fan will only suck out 800 cubic feet per minute and will only run while the sun is shining. Some people have reported that the solar panels on solar attic fans burn out after a few years, as well — does anyone have experience with these?
One last note: If you decide to add some attics fans, remember that working on your roof can be incredibly dangerous. Even more dangerous can be working in the attic in the summer — it is very easy to get heatstroke working in the attic. Be warned!
Rather than installing an attic fan, I would rather invest the money in added insulation for the attic floor. This reduces the heat transfer from a hot attic to the living spaces in the summer - and reduces heat loss in the winter. Also, if there are air leaks between the attic and the conditioned house, the attic fan will be pulling some of its air from the the house. That air - which many of us have paid to condition - will be replaced by hot, humid outside air. Most houses have LOTS of air leaks between the living space and the attic: every where there's a penetration through the drywall in an insulated ceiling - as well as leaks through the framing. Of course every house is different. If you have a lot of ductwork in the attic, keeping it cooler will have a big payoff. But first I would add a radiant barrier (in a hot climate), plug up duct leaks, and add insulation to the entire duct system. This is a complicated issue - affecting many of the home's systems. Consider a professional energy audit.