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.
EasyCron - Get frustrated with Cron on your server? Hosting limits your Cron use?
Upzilla.co - Upzilla.co alerts you when your websites, services, or applications have some issues with availability or performance. Quick and simple setup, multiple notification methods, and it's FREE.
crontab guru - The quick and simple editor for cron schedule expressions by Cronitor
UptimeRobot - Free Website Uptime Monitoring
Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.