MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
CleanMem - CleanMem keeps memory use in check on the system without the memory being pushed to the page file.
iStat Menus - "An advanced Mac system monitor for your menubar."
Wise Memory Optimizer - Wise Memory Optimizer a simple and effective RAM booster to help you free up memory and get your computer running smoothly again.
Stats - Simple macOS system monitor in your menu bar.
Mem Reduct - Mem Reduct is a small application that allows you to monitor and clean your computers memory.