SpeedFan is a program that monitors voltages, fan speeds and temperatures in computers with hardware monitor chips. SpeedFan can even access S.M.A.R.T. info 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).
SpeedFan can access digital temperature sensors and can change fan speeds accordingly, thus reducing noise.
SpeedFan can find almost any hardware monitor chip connected to the 2-wire SMBus (System Management Bus (trademark belonging to SMIF, Inc.), a subset of the I2C protocol) and works fine with Windows 9x, ME, NT, 2000, 2003, XP and Windows Vista. It works with Windows 64 bit too.