iStat Menus - "An advanced Mac system monitor for your menubar."
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
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…
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
Zabbix - Track, record, alert and visualize performance and availability of IT resources