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.
EasyCron - Get frustrated with Cron on your server? Hosting limits your Cron use?
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.
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.