Adam Weinroth recently posted an outstanding email interview with Clay Shirky and Jon Lebkowsky on tagging. I particularly liked Shirky's take on how tags are helping us navigate the transition from information scarcity to information abundance:
We have come from a land of information scarcity, and the assumptions from that world break in this one. In a world of information scarcity, the assumption is that after a user has some piece of information, it's problem solved. Once the library has given the patron the book, they should have no trouble keeping track of it.
In a world of information abundance, though, it's merely problem transferred, because now the user has so much information that they have become an accidental archivist, and everyone has the same two, bad strategies: get rigid about a categorization scheme for local files, and use some combination of bookmarks and common search strategies. (How often have you had to re-find something by recreating a particular Google search?)
Tags put post-industrial strength tools in the hands of ordinary users, allowing them to manage the increasingly large corpus of things they've already found once, allowing them to re-find them easily.
Hat tip to Marshall.