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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
UptimeRobot - Free Website Uptime Monitoring
updown.io - updown.io is a HTTP(S)/TCP/ICMP monitoring service which checks periodically the URL you want and reports back any anomaly.
Better Uptime - We call you when your website goes down
Freshping - Application and Data, Monitoring, and Website Monitoring