UptimeRobot - Free Website Uptime Monitoring
Uptime Kuma - A fancy self-hosted monitoring tool.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
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.