Excipio!

Rollbar collects errors that happen in your application, notifies you, and analyzes them so you can debug and fix them. JavaScript, Ruby, Python, PHP, Node.js, Android, Flash, .NET, and Java libraries are available.

Airbrake collects errors generated by other applications, and aggregates the results for review.

God mode in Production code. OverOps tells developers why code crashes and threads freeze. Server debugging doesn't have to suck.

SessionStack helps you see your web app defects through a user’s eyes.

log4net is a tool to help the programmer output log statements to a variety of output targets. log4net is a port of the excellent log4j framework to the .NET runtime. We have kept the framework similar in spirit to the original log4j while taking advantage of new features in the .NET runtime. For more information on log4net see the features document.

Error tracking, uptime monitoring, and performance metrics you'll love.

May be the coolest cloud based error logger for .NET web applications. Based on the de-facto standard error component ELMAH, logging errors from your webserver requires nothing more than installing a NuGet package.

Monitoring for Ruby developers. All the metrics and alerts you need to keep your app running smooth. Support for Rails, Rack, Sinatra, Padrino & Grape.

Honeybadger is the modern error management service for rails

ELMAH (Error Logging Modules and Handlers) is an application-wide error logging facility that is completely pluggable. It can be dynamically added to a running ASP.NET web application, or even all ASP.NET web applications on a machine, without any need for re-compilation or re-deployment.

Doctor Dump is a free crash reporting and memory dump analyzing service. Its work is to collect software problem reports (crashes, failures, assert violations and so on) from your customers all over the world, to group them and to give you all necessary data to fix the problem. Also it is possible to provide users with solution to the problem immediately after the crash.

A lightweight logging framework for .NET 4.0+.