Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Pingbreak - Pingbreak is an original and free monitoring service.
Cronly - Keep track of your cron jobs and SSL certificates. Don't let them fail unnoticed.
DownDetector - A simple service to tell you whether a website is down.
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
UptimeMate - UptimeMate checks your website for uptime, speed and health 24/7.