Is there a user-friendly Log4Net log files viewer?

This one looks promising:
http://yourlog4netviewer.codeplex.com/

Saw this one as well:
http://logexpert.codeplex.com/

Edited: this one as well:
http://insightextensions.codeplex.com/

Try GamutLogViewer.

GamutLogViewer© is log file, logfile, viewer that works with Log4J, Log4Net, NLog, and user defined formats including ColdFusion. It supports filtering, searching, highlighting and many other useful features. This is a Windows application.

BareTail is a very nice tool to monitor log files.

You can monitor multiple log files, and the Pro version supports search.