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.
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
DownDetector - A simple service to tell you whether a website is down.
UptimeMate - UptimeMate checks your website for uptime, speed and health 24/7.