xScan - xScan is an application for viewing the behavior of your computer and Mac.
SpeedFan - Hardware monitor for Windows that can access digital temperature sensors located on several 2-wire SMBus Serial Bus. Can access voltages and fan speeds and control fan speeds. Includes technical articles and docs.
Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
smcFanControl - [Download] smcFanControl 2.
iMac HDD Fan Control - iMac HDD Fan Control is an HDD fan control for the Mac operating systems by using which the Mac users can control the speed and noise of the fan of the Mac.
Argus Monitor - Argus Monitor is for monitoring and analyzing the temperature and the health status of the hardware parts of the system.