MenuBar Stats - Advanced Mac system monitor right from the menu bar
Stats - Simple macOS system monitor in your menu bar.
iStat Menus - "An advanced Mac system monitor for your menubar."
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
taskbar-stats - Display live system stats on Windows’s taskbar
Rooster - Local sharing community where neighbors share free resources