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.
Cronly - Keep track of your cron jobs and SSL certificates. Don't let them fail unnoticed.
WDT.io - WDT.io is a monitoring-as-a-service solution for cron jobs and servers.
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.
PushMon - PushMon is a new kind of cloud-based monitoring system.