SpeedFan is a freeware program that monitors voltages, fan speeds and temperatures in computers with hardware monitor chips. SpeedFan can even access S.M.A.R.T. info for those hard disks that support this feature and show hard disk temperatures. SpeedFan supports SCSI disks too. SpeedFan can even change the FSB on some hardware (but this should be considered a bonus feature).<br />
At the lowest level, SpeedFan is a hardware monitor software that can access temperature sensors, but its main feature is that it can change fan speeds (depending on the capabilities of your sensor chip and your hardware) according to the temperatures inside your PC, thus reducing noise and power consumption.