UptimeRobot - Free Website Uptime Monitoring
Is It Down Right Now? - Network & Admin
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
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.