SpeedFan monitors fan speeds, temperatures and voltages in computers with hardware monitor chips. It can even access S.M.A.R.T. info for those hard disks that support this feature and show hard disk temperatures too, if supported. It can even change the FSB on some hardware. At the lowest level, it is a hardware monitor software, but its main feature is that it can control fan speeds according to the temperatures inside your pc, thus reducing noise and power consumption.

http://www.almico.com/speedfan.php