Selenium - Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that.

Exceptionless - Don't let your users walk away in frustration.

DalekJS - DalekJS is an open source UI testing tool.

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.

Webinject - WebInject is a free tool for automated testing of web applications and web services.

RoRvsWild - All-in-one monitoring for Ruby on Rails applications. Track performances & errors for requests & background jobs.