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.
Pingbreak - Pingbreak is an original and free monitoring service.
Cronotary - Painless periodical tasks monitoring. Cronotary makes sure your background jobs are up and running.
DownDetector - A simple service to tell you whether a website is down.
Cronly - Keep track of your cron jobs and SSL certificates. Don't let them fail unnoticed.