Quiet Fan implements a control loop to set your CPU or motherboard fan speed. This means that it uses the lowest possible fan speed to keep a target temperature, and eliminates damaging hardware temperature oscillations.
This software wants to be the Unix brother of Windows SpeedFan. Using the package lm-sensors it is able to monitor the system temperatures and the fan speeds and control them by easy commands. It's being planned to add a user GUI to be more friendly.