OnlineOrNot - Uptime monitoring for software teams (even teams of one).
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Uptime Kuma - A fancy self-hosted monitoring tool.
Server Monitoring - A server monitor is a tool that monitors your server and website on close intervals and reports to you if it's inaccessible.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.