Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
UptimeRobot - Free Website Uptime Monitoring
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Uptime Kuma - A fancy self-hosted monitoring tool.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring