Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
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.
GPU-Z - GPU-Z is a lightweight system utility designed to provide vital information about your video card...
Sidebar Diagnostics - SidebarDiagnostics - A simple sidebar for Windows desktop that displays hardware diagnostic information.
iStat Menus - "An advanced Mac system monitor for your menubar."
GSmartControl - Hard disk drive health inspection tool