Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.
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.
Robodash - Cron and uptime monitoring dashboard
UptimeRobot - Free Website Uptime Monitoring
BurnoutIndex - Check your burnout risk. Backed by science, forever-free.