Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
iStat Menus - "An advanced Mac system monitor for your menubar."
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.
Stats - Simple macOS system monitor in your menu bar.
Core Temp - Nov 4, 2017 - Core Temp is a compact, no fuss, small footprint, yet powerful program to monitor processor temperature and other vital information.
MenuBar Stats - Advanced Mac system monitor right from the menu bar