Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Site Down - Site down outage and downtime reports from SiteDown.co
DownDetector - A simple service to tell you whether a website is down.
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
Pingbreak - Pingbreak is an original and free monitoring service.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.