NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
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.
Server Monitoring - A server monitor is a tool that monitors your server and website on close intervals and reports to you if it's inaccessible.
CatchJS - Unbloated, yet feature rich. CatchJS provides JavaScript Error Tracking the way it should be.
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.
ErrLytics - Javascript error reporting and performance monitoring tool