SpeedFan 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 too, if supported. SpeedFan fully supports SCSI disks too. SpeedFan can even change the FSB on some hardware (but this should be considered a bonus feature). At the lowest level, SpeedFan is an hardware monitor software that can access temperature sensors, but its main feature is that it can control 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.
Version 4.22 added, fixed, and verified fan speed changing support for a lot of chips and motherboards; fixed temperature readings for SCSI disks; added full support for Fintek chips; added Japanese translation and several other fixes and improvements.