UptimeRobot - Free Website Uptime Monitoring
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Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Site Down - Site down outage and downtime reports from SiteDown.co
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.