MHDD - MHDD is the most popular freeware program for low-level HDD diagnostics.
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.
HDD Regenerator - HDD Regenerator is a unique program for regeneration of physically damaged hard disk drives.
Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
Hard Disk Sentinel - Monitoring hard disk health and temperature. Test and repair HDD problems and predict failures.
Argus Monitor - Argus Monitor is for monitoring and analyzing the temperature and the health status of the hardware parts of the system.