Pingbreak - Pingbreak is an original and free monitoring service.
DownDetector - A simple service to tell you whether a website is down.
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.
UptimeRobot - Free Website Uptime Monitoring
Screpy - Screpy is a web analysis tool that can analyze all pages of your websites in one dashboard and monitor them with your team. It's powered by Lighthouse and it also includes some different analysis tools (SEO, SERP, W3C, Uptime, etc).
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.