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
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
Is it up? - A simple tool to check if a website or ip address is up or down.