Overview of to Enterprise Library Logging Application Block and Alternatives
Loren Halvorson's comparision of the Enterprise Library Logging & Instrumentation Application Block and Log4Net (Published in 2005)
Comments on the EntLib/Log4Net feature and performance comparison (Published in 2005)
EntLib v4 has a few performance improvements in Logging Block.
Based on quick read I feel that Log4Net and NLog are simple and may be sufficient for small applications.
But they seems do not support specifying event Categories, that allow to separate messages from different part of application. So I prefer Enterprise Library Logging Application Block (LAB) for our app.
The discussion on stackOverview voted for Log4Net.
It also recommends http://www.theobjectguy.com/DotNetLog/compare.aspx