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.
Argus Monitor - Argus Monitor is for monitoring and analyzing the temperature and the health status of the hardware parts of the system.
HWMonitor - HWMonitor is a hardware monitoring program that reads PC systems main health sensors : voltages, temperatures, fans speed.
GPU-Z - GPU-Z is a lightweight system utility designed to provide vital information about your video card...
smcFanControl - [Download] smcFanControl 2.