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.
Venmon - Compact size system resources monitoring utility and multi-ping tool for Windows
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.
Scout - Scout ♥ monoliths.
Checkmk - Checkmk - the software for effective IT monitoring
Pingbreak - Pingbreak is an original and free monitoring service.