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)
smcFanControl - [Download] smcFanControl 2.
OSX Activity Monitor - Activity Monitor shows the processes that are running on your Mac, so you can manage them and see how they affect your Mac's activity and performance.
iPulse - Using its concise and appealing user interface, iPulse graphically displays the inner workings of macOS on you desktop, menu bar or Dock.