Is It Down Right Now? - Network & Admin
Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Is it up? - A simple tool to check if a website or ip address is up or down.
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.