BitBar - Put anything in your Mac OS X menu bar
SwiftBar - Powerful macOS menu bar customization tool.
iStat Menus - "An advanced Mac system monitor for your menubar."
Stats - Simple macOS system monitor in your menu bar.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
eul - System & Hardware and OS & Utilities