Is It Down Right Now? - Network & Admin
Uptime Kuma - A fancy self-hosted monitoring tool.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
UptimeRobot - Free Website Uptime Monitoring