Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
DownDetector - A simple service to tell you whether a website is down.
Upzilla.co - Upzilla.co alerts you when your websites, services, or applications have some issues with availability or performance. Quick and simple setup, multiple notification methods, and it's FREE.
Pingbreak - Pingbreak is an original and free monitoring service.
UptimeRobot - Free Website Uptime Monitoring