Clay Shirky has a great article up about the history of classifications of things (ontologies).
With movements afoot on the great Semantic Web, which seem to be way too bogged down in theory without significant practical applications, he asks the question of “Where to now?”
His theory is that the concept of tagging things (much like a blog post or a del.icio.us link) will take us to the next level of knowledge integration. Search having been the light to come out of the categorisation of old.
As an aside, it is interesting how the categorisation of things follows the Long Tail. And it is the ability of tagging to support the long tail that will help it survive.