Opbeat - Application monitoring for developers
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.
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Atatus - Full Stack Observability Platform
Better Uptime - We call you when your website goes down