Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.
Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Dead Man's Snitch - Monitor Cron Jobs and Windows Scheduled Tasks with ease.
DownDetector - A simple service to tell you whether a website is down.
Robodash - Cron and uptime monitoring dashboard
Pingbreak - Pingbreak is an original and free monitoring service.