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.
Dead Man's Snitch - Monitor Cron Jobs and Windows Scheduled Tasks with ease.
Cronotary - Painless periodical tasks monitoring. Cronotary makes sure your background jobs are up and running.
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.
Cronly - Keep track of your cron jobs and SSL certificates. Don't let them fail unnoticed.