I just started using Log4Net and was looking to see what you have found to be useful in your logging experiences.
What types of things have you found to be useful to log
There's another log4net log viewer (other than the Apache Chainsaw) which my company has been using for some time, it's called "log4net Dashboard" and is being developed by a Norwegian company (I think) called FaktNet, it's website is http://www.l4ndash.com.
It provides a web based dashboard which is quite intuitive and provides a good overview on the log, can be used with many different appenders (such as rolling file or SQL-server appender) but it isn't free as the Apache Chainsaw is. They do have a developer's license though, which is free and allows local use only, which would be sufficient for example for a freelancer wanting to monitor his sites (a single l4n-dashboard can connect to multiple log-sources).
FaktNet has various licenses, depending on how many users are supposed to be able to access the dashboard, and their Enterprise license isn't really expensive (it's $600 I think). Given its ability to access multiple logs it can be a major asset to a development team that's developing and monitoring many medium to large scale websites.
Another really usefull apender is the DBAppender
which may log info in a Database, which is obviously incredibly useful so as to query a log.
More info on this in this article.