Is It Down Right Now? - Network & Admin
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.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
Better Uptime - We call you when your website goes down
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
UptimeRobot - Free Website Uptime Monitoring