iStat Menus - "An advanced Mac system monitor for your menubar."
Process Tamer - Freeware Microsoft Windows utility that monitors the CPU use of applications, alerts when they use too much, and reduces their priority to keep your PC fast.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
Big Meter Pro - Big Meter Pro is a utility that monitors the disk and memory of your CPU.
Stats - Simple macOS system monitor in your menu bar.
SysGauge - SysGuage is a responsive software utility that monitors the memory status, CPU usage, as well as network and disk activities of your system.