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.
Cronhub - Cronhub helps you to easily monitor all your cron jobs in a beautiful dashboard. It alerts you when your cron job doesn't run on time or it fails.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
BurnoutIndex - Check your burnout risk. Backed by science, forever-free.
Better Uptime - We call you when your website goes down