IsZDown - Monitor the errors and status of the most popular websites and apps. Check whether they are down right now or not.
Pingbreak - Pingbreak is an original and free monitoring service.
DownDetector - A simple service to tell you whether a website is down.
IsItDown RightNow - "Is It Down Right Now" monitors the status of your favorite web sites and checks whether they are down or not.
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.
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.