iStat Menus - "An advanced Mac system monitor for your menubar."
Zabbix - Track, record, alert and visualize performance and availability of IT resources
iStatistica Pro - System monitor. Reinvented. 💻 🖥
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.
Stats - Simple macOS system monitor in your menu bar.
Prometheus - An open-source systems monitoring and alerting toolkit.