UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
UptimeRobot - Free Website Uptime Monitoring
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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.
Site Down - Site down outage and downtime reports from SiteDown.co
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring