Eureka is a set of tools which allow you to store data from HTTP logs in a database, then use a web interface to produce customised reports and "drill down" to find out more information about certain things.
For instance, a report which showed the number of requests for HTML pages might have a "Who" link next to each page's request count, which would link to a list of who's been requesting that page. Likewise, a "Whence" link would show the referring sites which sent people to that page.
Eureka is written in Perl and uses DBI to interact with any of a number of database servers management. (Note that the number is currently 1, and the database in question is MySQL.)
Eureka's license is the Netizen Open Source License (NOSL) which is basically the MPL with Australian-specific bits in it. This means that Eureka is Open Source software as specified by the Open Source Initiative's Open Source Definition
Demo (on about 2 weeks of Netizen's logs)