UptimeRobot - Free Website Uptime Monitoring
Is It Down Right Now? - Network & Admin
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
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.