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.
DownDetector - A simple service to tell you whether a website is down.
EasyCron - Get frustrated with Cron on your server? Hosting limits your Cron use?
Pingbreak - Pingbreak is an original and free monitoring service.
crontab guru - The quick and simple editor for cron schedule expressions by Cronitor