Is It Down Right Now? - Network & Admin
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
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.
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.