StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime 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.
UptimeRobot - Free Website Uptime Monitoring
Uptime Kuma - A fancy self-hosted monitoring tool.
Monitis - Monitis specializes in IT monitoring, covering server, website, app monitoring and more within an all-in-one monitoring platform.
Freshping - Application and Data, Monitoring, and Website Monitoring