The slogan package
Slogan is the first (that I know of) web log analyser to work interactively. Instead of generating a batch of static pages containing pretty graphs for Marketing, it allows you to frame your own queries and get the answers back in real time.
Its unique (again: as far as I know) hierarchical query interface allows you to drill down and make more specific queries as you investigate what your server has been up to.
Latest [dogfood] version
Version 0.05: package (zip), package (gzipped tar). Licence: new-BSD.
Slogan is still in an early stage of development, and is currently suffering from lack of time on my behalf. It is in active use on a number of servers but not all features are ready and there is almost no documentation. You will also need a CGI-capable web server to run its web-based query interface (ASP and stand-alone-server versions will follow), and a database to store the logs in (currently, access through PygreSQL, MySQLdb and Windows ODBC driver modules is supported).