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."
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.
iStatistica Pro - System monitor. Reinvented. 💻 🖥
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.