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.
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.
Site Down - Site down outage and downtime reports from SiteDown.co