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