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.
Serilog - Backend Development and Utilities
AirBrake - Airbrake is the leading exception reporting service, currently providing error tracking.
log4net - log4net is a tool to help the programmer output log statements to a variety of output targets.
Raygun - Raygun gives developers meaningful insights into problems affecting their applications. Discover issues - Understand the problem - Fix things faster.