Uptime Kuma - A fancy self-hosted monitoring tool.
UptimeRobot - Free Website Uptime 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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Statping - Status Page for monitoring your websites and applications with beautiful graphs, analytics, and plugins. Run on any type of environment.