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.

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.

CatchJS - Unbloated, yet feature rich. CatchJS provides JavaScript Error Tracking the way it should be.

Raygun - Raygun gives developers meaningful insights into problems affecting their applications. Discover issues - Understand the problem - Fix things faster.

ErrLytics - Javascript error reporting and performance monitoring tool

GlitchTip - GlitchTip is an open source, Sentry API compatible error tracking platform.