smcFanControl is a quite curious application that allows you to chane the speed of the different fans inhabiting your computer.
It allows you to set the RPM (Revolutions per minute)for each one so you can reduce the temperature inside the computer.
That task is usually done by the computer automatically, but in case you think it is failing, you can use smcFanControl and be sure everything is well refrigerated.
It can be used to minimize the sound produced y the fans at night by turning them off too, but be careful if you do this, because the temperature can kill your computer.
Finally, you can set different speeds for each fan.