Little Snitch - Little Snitch is a firewall application that monitors and controls outbound internet traffic.
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.
NetGuard - NetGuard
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Portmaster - Is a Privacy Modular Privacy App which includes a "DNS over TLS" feature as well as an "Application Firewall".
Better Uptime - We call you when your website goes down