DownNotifier.com - DownNotifier knows about website availability and provides notification.
UptimeRobot - Free Website Uptime Monitoring
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
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.
Is it up? - A simple tool to check if a website or ip address is up or down.