UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
DownNotifier.com - DownNotifier knows about website availability and provides notification.
Is It Down Right Now? - Network & Admin
UptimeRobot - Free Website Uptime Monitoring
Site Down - Site down outage and downtime reports from SiteDown.co
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.