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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.
Uptime Kuma - A fancy self-hosted monitoring tool.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Better Uptime - We call you when your website goes down
Freshping - Application and Data, Monitoring, and Website Monitoring