Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.
UptimeRobot - Free Website Uptime Monitoring
Dead Man's Snitch - Monitor Cron Jobs and Windows Scheduled Tasks with ease.
Uptime Kuma - A fancy self-hosted monitoring tool.
Cronhub - Cronhub helps you to easily monitor all your cron jobs in a beautiful dashboard. It alerts you when your cron job doesn't run on time or it fails.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.