DownDetector - A simple service to tell you whether a website is down.
Pingbreak - Pingbreak is an original and free monitoring service.
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).
Montastic - Receive instant alert notification when your website goes down. Easy to use user interface.