log4net is described as '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'. There are six alternatives to log4net for a variety of platforms, including Mac, Windows, .NET Framework, Linux and Online / Web-based. The best alternative is Raygun. It's not free, so if you're looking for a free alternative, you could try NLog or ELMAH. Other great apps like log4net are elmah.io (Paid), OneTrueError (Free Personal) and NLogEx (Free, Open Source).
NLog is a free logging platform for .NET with rich log routing and management capabilities. It makes it easy to produce and manage high-quality logs for your application regardless of its size or complexity.
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.
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.