UptimeRobot - Free Website Uptime Monitoring
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
Uptime Kuma - A fancy self-hosted monitoring tool.
Uptime Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
AppBeat - AppBeat is reliable website monitoring service that alerts you when there is a downtime or failure. It also measures performance and provides functional tests.