UptimeRobot - Free Website Uptime Monitoring
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Uptime Kuma - A fancy self-hosted monitoring tool.