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Stats - Simple macOS system monitor in your menu bar.
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.
iStat Menus - "An advanced Mac system monitor for your menubar."
Argus Monitor - Argus Monitor is for monitoring and analyzing the temperature and the health status of the hardware parts of the system.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.