Rollbar - Rollbar collects errors that happen in your application, notifies you, and analyzes them so you can debug and fix them. Ruby, Python, PHP, Node.js, JavaScript, and Flash libraries available.
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.
CatchJS - Unbloated, yet feature rich. CatchJS provides JavaScript Error Tracking the way it should be.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
ErrLytics - Javascript error reporting and performance monitoring tool
Better Uptime - We call you when your website goes down