Stats - Simple macOS system monitor in your menu bar.
ZetaWatch - Manage ZFS volumes in the menu bar
iStat Menus - "An advanced Mac system monitor for your menubar."
Baobab - Distributor of workload automation, financial statement reporting, data access, IT Asset Management and Service Desk software solutions.
eul - System & Hardware and OS & Utilities
Filelight - Filelight is an application to visualize the disk usage on your computer.