iStat Menus - "An advanced Mac system monitor for your menubar."
SysGauge - SysGuage is a responsive software utility that monitors the memory status, CPU usage, as well as network and disk activities of your system.
Process Explorer - The top window always shows a list of the currently active processes, including the names of their owning accounts, whereas the information displayed in the bottom window depends on the mode that Process Explorer is in: if it is in handle mode you'l…
Big Meter Pro - Big Meter Pro is a utility that monitors the disk and memory of your CPU.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
AMD64 CPU Assistant - AMD64 CPU Assistant