Note: This review addresses v4.33 of the software.
This is a must-download for serious overclockers. While SpeedFan touts itself as a monitor for your system's temperature and fan speed, the real reason it's worthwhile is that it lets you easily overclock your PC and then adjust the fan speed so it doesn't overheat.
Note: This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.