Vigil - Vigil offers website-monitoring and downtime notifications.
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.
Grafana Cloud - Your observability stack, managed by us completely for free
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
Better Uptime - We call you when your website goes down