UptimeRobot - Free Website Uptime Monitoring
Is It Down Right Now? - Network & Admin
Uptime Kuma - A fancy self-hosted monitoring tool.
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Site Down - Site down outage and downtime reports from SiteDown.co