Sentry.io - From error tracking to performance monitoring, developers can see what actually matters, solve quicker, and learn continuously about their applications - from the frontend to the backend.
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.
AirBrake - Airbrake is the leading exception reporting service, currently providing error tracking.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
TrackJS - JavaScript Error Tracking for Modern Web Applications
Better Uptime - We call you when your website goes down