The swamp cooler method is cheap and effective. That is, put the carboy in a tub of water, and drape a wet towel towel over it. For added measure, you can point a fan at it. To go even further, buy a cheap aquarium pump and run tubing from the pump to the top of the towel (keeping it wet, so the fan doesn't dry it out - giving you constant evaporative cooling).
Beyond that there are million ways to ferment, ranging from utter ghetto, to ridiculously expensive.