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.
AppBeat - AppBeat is reliable website monitoring service that alerts you when there is a downtime or failure. It also measures performance and provides functional tests.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
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.