StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
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.
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
RPi-Monitor - RPi-Monitor is a tool that provides all the connected system peripherals properties and current metrics.
UptimeBar - A simple OS X menu bar app for monitoring website uptime
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.