Uptime Kuma - A fancy self-hosted monitoring tool.
Pingbreak - Pingbreak is an original and free monitoring service.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
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
DownDetector - A simple service to tell you whether a website is down.