WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
UptimeRobot - Free Website Uptime Monitoring
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Uptime Kuma - A fancy self-hosted monitoring tool.
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.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
OnlineOrNot - Uptime monitoring for software teams (even teams of one).