Zabbix - Track, record, alert and visualize performance and availability of IT resources
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Paessler PRTG - An advanced network monitoring solution to monitor network up/downtime, traffic and usage.
RPi-Monitor - RPi-Monitor is a tool that provides all the connected system peripherals properties and current metrics.
Observium - Observium
MenuBar Statistics - MenuBar Statistics is a lightweight app that sits quietly in your menu bar of macOS and displays computer information such as CPU, Disk, and Network, which is updated every 5 seconds.