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.
Is It Down Right Now? - Network & Admin
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
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.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.