DownNotifier.com - DownNotifier knows about website availability and provides notification.
Is It Down Right Now? - Network & Admin
UptimeRobot - Free Website Uptime Monitoring
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
Is it up? - A simple tool to check if a website or ip address is up or down.