Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Is It Down Right Now? - Network & Admin
Pingbreak - Pingbreak is an original and free monitoring service.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
DownDetector - A simple service to tell you whether a website is down.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.