Stats - Simple macOS system monitor in your menu bar.
iStat Menus - "An advanced Mac system monitor for your menubar."
eul - System & Hardware and OS & Utilities
OS Track - Keeps track of system resources.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.