ELMAH is described as '(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'. There are six alternatives to ELMAH for a variety of platforms, including Windows, Mac, Online / Web-based, PHP and Linux. The best alternative is Raygun. It's not free, so if you're looking for a free alternative, you could try log4net or NLogEx. Other great apps like ELMAH are elmah.io (Paid), Stackify (Paid) and OneTrueError (Free Personal).
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.