I have been talking a great deal about Enterprise Library 2.0 Logging Application Block in the following 4 posts:
but it obviously isn't the only game in town.
CodeProject has a newly updated post on NLog, which is another .NET logging library that I mentioned awhile ago. I haven't tried it out, but it appears to be similar to log4net.
I have used and continue to use log4net in some of my projects and it is an excellent logging library for .NET.