UptimeRobot - Free Website Uptime Monitoring
Uptime Kuma - A fancy self-hosted monitoring tool.
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
Statping - Status Page for monitoring your websites and applications with beautiful graphs, analytics, and plugins. Run on any type of environment.
TLS Checker - TLS Checker is a Free TLS/SSL Expiry Monitoring Service which includes Discovery, Inventory and Management of certificates.