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 too, if supported. SpeedFan 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 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. Several sensors, like Winbond's and the AS99127F support fan speed changing, as well as others from Maxim, Myson, Analog Devices, National Semiconductor and ITE, but the hardware manufacturer must have connected the relevant pins to some additional, yet trivial, circuitry. This means that if you have, say, a Winbond W83782D on a BP6 then you're ok, but not every motherboard with such an hardware monitor chip will be able to change fan speeds. From one of the very first hardware monitor chips that could be found in standard PCs, the National Semiconductor LM75 (and all of its clones, like the Philips NE1617 and the Philips NE1618 or the Maxim MAX1617) or the Analog Devices ADM1021, such chips have been greatly improved, both in their precision and in their capabilities. Current chips can monitor fan speeds, voltages and change fan speeds bu using PWMs (Pulse Width Modulation). Some chips can even be programmed to vary fan speeds without any additional software intervention. If your BIOS was programmed to setup such chips this way you can still try to use SpeedFan's Advanced Configuration to revert to manual (software controlled) mode. Winbond W83697HF, Analog Devices ADT7463, SMSC EMC6D102, ITE IT8712F, National LM85C and Maxim MAX6650 are very good candidates. Some SuperIO chips include temperature sensors too. SpeedFan can automatically detect them and use their features. SpeedFan can find almost any hardware monitor chip connected to the 2-wire SMBus (System Management Bus, a subset of the I2C BUS) Serial Interface and to the ISA BUS and works fine with Windows 9x, ME, NT, 2000, 2003 and Windows XP. It can be minimized to the tray and is compatible with Motherboard Monitor 5.

Some words about Hmonitor from the Net:
"Hardware sensors monitor is another program nobody should be without. This program is one of the best hardware sensor programs that allows you to check the temperature of any component on your computer with a sensor. It also has alarms that can be set when the temperature goes too high and even has built in software cooling."
Keith "Farrel" McClellan

Hardware sensors monitor utilizes sensor chips on smart motherboards to track system and CPU temperatures, voltages and cooling fans. By quickly catching an overheated computer, you can prevent disasters such as system instability or damage. The program is resource friendly, resides quietly in the system tray, and continually monitors the various parameters. By clicking its icon, you'll view real time values for each component. Hmonitor supports all Windows OS: Windows 9X/ME/NT/2000/XP/2003, inluding X64/EM64T editions.
Once it has detected an overheating state, Hmonitor can perform a number of tasks. First, it can sound an audio alarm, then shut down the system or optionally execute any selected application (i.e. sending E-mail alert messages to system administrator). It can also log information each minute to a file on your hard drive. From the settings dialog, Hmonitor enables you to set "yellow zone" and "red zone" temperatures for the motherboard and CPU. You can also set minimum fan RPM before a warning is issued. In addition, nominal values and thresholds can be adjusted for voltage parameters.