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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."
smcFanControl - [Download] smcFanControl 2.
iStatistica Pro - System monitor. Reinvented. 💻 🖥
Argus Monitor - Argus Monitor is for monitoring and analyzing the temperature and the health status of the hardware parts of the system.
Stats - Simple macOS system monitor in your menu bar.