Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
Cronotary - Painless periodical tasks monitoring. Cronotary makes sure your background jobs are up and running.
Cronly - Keep track of your cron jobs and SSL certificates. Don't let them fail unnoticed.