Uptime Kuma - A fancy self-hosted monitoring tool.
UptimeRobot - Free Website Uptime Monitoring
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
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.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!