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
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.
Site Down - Site down outage and downtime reports from SiteDown.co
UptimeRobot - Free Website Uptime Monitoring
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.