OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Uptime Kuma - A fancy self-hosted monitoring tool.
HetrixTools - FREE Uptime Monitoring + Server Monitoring + Status Page + Blacklist Monitoring!
UptimeRobot - Free Website Uptime Monitoring