SpeedFan monitors voltages, fan speeds, and temperatures in computers with hardware monitor chips. It can even access SMART information for hard disks that support this feature, and show hard-disk temperatures if that's supported. SpeedFan fully supports SCSI disks and can even change the FSB on some hardware (but this should be considered a bonus feature). At the lowest level, SpeedFan is a hardware monitor software that can access temperature sensors, but mainly 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 4.24 adds µGuru support, a new Get Config facility, new translations, improved support for known hardware, and several minor fixes and enhancements.