Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Pingbreak - Pingbreak is an original and free monitoring service.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.
Uptime Kuma - A fancy self-hosted monitoring tool.