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.
UptimeRobot - Free Website Uptime Monitoring
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.