WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Cronly - Keep track of your cron jobs and SSL certificates. Don't let them fail unnoticed.
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.