An all inclusive web monitoring service.
DownDetector - A simple service to tell you whether a website is down.
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.
Outage.Report - See if your provider or web service is having an outage or it's just you. Post yours and see other's reports and complaints
UptimeRobot - Free Website Uptime Monitoring
IsItDown RightNow - "Is It Down Right Now" monitors the status of your favorite web sites and checks whether they are 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.