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.
Statping - Status Page for monitoring your websites and applications with beautiful graphs, analytics, and plugins. Run on any type of environment.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
LambStatus - Serverless status page system which enables you to build a status page with minimum effort and cost!
Better Uptime - We call you when your website goes down
cState - 🔥 Free & open source under MIT!