I have just finished redesigning the Debtags website, consolidating a 9 years long history of bringing categories to a large and complex distribution like Debian. There is more to Debtags than meets the eye: interesting library science concepts, anonymous submissions, an efficient review workflow, a fully automated tagging engine, dynamically generated tagging hints, fancy tag search algorithms. On top of that, there is not much that is Debian-specific in the new codebase, so if you are interested in implementing categories on your distribution of choice, this could give you quite a head start.
Speakers: Enrico Zini