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.
Hardinfo - Hardinfo is a system information and benchmark tool for Linux.
Macs Fan Control - Macs Fan Control is an iMac, MacBook Pro, MacBook Air, Mac Mini and Mac Pro fan control that is the solution of two basic issues namely noise problems and overheating problems.
CPU-Z - CPU-Z is a freeware that gathers information on some of the main devices of your system : Processor name and number, codename, process, package, cache levels.
xScan - xScan is an application for viewing the behavior of your computer and Mac.