Vigil - Vigil offers website-monitoring and downtime notifications.
UptimeRobot - Free Website Uptime Monitoring
Grafana Cloud - Your observability stack, managed by us completely for free
AppBeat - AppBeat is reliable website monitoring service that alerts you when there is a downtime or failure. It also measures performance and provides functional tests.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
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.