SpeedFan is a freeware program that monitors fan speeds, temperatures and voltages in computers with hardware monitoring chips. SpeedFan can even access S.M.A.R.T. info for those hards disks that support this feature (almost all ) and show hard disk temperatures too, if supported. SpeedFan can even change the FSB on some motherboards. At the lowest level, SpeedFan is an hardware monitoring software, but its main feature is that it can control the speed of the fans (depending on your sensor chip) according to the temperatures inside your pc, thus reducing noise and power consumption.
Most Winbond sensors and ASUS AS99127F support fan speed changing, as well as others from MYSON, ANALOG DEVICES, NATIONAL SEMICONDUCTORS and ITE, but the motherboard should make use of available pins. This means that if you have, say, a W83782D on a BP6 then you're ok. If you have a W83782D on some GigaByte's... you're not
Note: This program is aimed to the power user. To the one who knows what he's doing