iStat Menus - "An advanced Mac system monitor for your menubar."
Stats - Simple macOS system monitor in your menu bar.
Beaconly - Flexible, easy to use data aggregating menu app that ties relevant information in a macOs menu bar.
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.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)