Mac OS X includes an automatic fan depending on the temperature of the machine. If your Mac seems overly warm, you can still exercise greater control over the fans with the appropriately named fancontrol.
Once installed, the application is lodged in System Preferences. Fancontrol does not replace the system completely automatic control of fans (which could cause major stability problems), but can still change the speed of the fan base, and the trigger of those latter . Three sliders are available for this, and a graph can observe their effect visually.