Is It Down Right Now? - Network & Admin
DownNotifier.com - DownNotifier knows about website availability and provides notification.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
UptimeRobot - Free Website Uptime Monitoring
Site Down - Site down outage and downtime reports from SiteDown.co
Is it up? - A simple tool to check if a website or ip address is up or down.