iStat Menus - "An advanced Mac system monitor for your menubar."
Stats - Simple macOS system monitor in your menu bar.
MenuBar Stats - Advanced Mac system monitor right from the menu bar
iStatistica Pro - System monitor. Reinvented. 💻 🖥
taskbar-stats - Display live system stats on Windows’s taskbar
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.