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
DownDetector - A simple service to tell you whether a website is down.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
Pingbreak - Pingbreak is an original and free monitoring service.
Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.