UptimeRobot - Free Website Uptime Monitoring
Is It Down Right Now? - Network & Admin
Uptime Kuma - A fancy self-hosted monitoring tool.
Site Down - Site down outage and downtime reports from SiteDown.co
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.