Testim - Stable, self-healing, end-to-end test automation via machine learning. Testim helps accelerate the delivery of high-quality software. Speed up test-authoring and improve the stability of automated, end-to-end tests.
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.
Selenium - Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that.
UptimeRobot - Free Website Uptime Monitoring
Ghost Inspector - Easily create automated browser tests for your websites and web apps. Ensure everything works and looks the way it should. No coding required. 14 day free trial!
Uptime Kuma - A fancy self-hosted monitoring tool.