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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UptimeRobot - Free Website Uptime Monitoring
Monitis - Monitis specializes in IT monitoring, covering server, website, app monitoring and more within an all-in-one monitoring platform.
Uptime Kuma - A fancy self-hosted monitoring tool.
Uptime.com - Everything you require for unified availability monitoring Simple & intuitive industry leading Enterprise-grade features delivered at a fair price, that are continuously improving.