Exceptionless - Don't let your users walk away in frustration.
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.
RoRvsWild - All-in-one monitoring for Ruby on Rails applications. Track performances & errors for requests & background jobs.
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.
TrackJS - JavaScript Error Tracking for Modern Web Applications
Xamarin Insights - Xamarin Insights is a real-time app monitoring platform.