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
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.
IsItDown RightNow - "Is It Down Right Now" monitors the status of your favorite web sites and checks whether they are down or not.
UptimeRobot - Free Website Uptime Monitoring