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.
Is It Down Right Now? - Network & Admin
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
Better Uptime - We call you when your website goes down
Site Down - Site down outage and downtime reports from SiteDown.co