I'm going to quote David's whole post because it has a bunch of good links.
I totally agree that this "ethnoclassification" is really an amazing solution to the metadata problem. Although, as they point out, there are some problems, I think that we'll find solutions. I feeling very taggy these days. I think there should be more cross-site tag linking. Blog categories, wiki pages, music meta data, and many other things can be "tagged". TAGCON 2005! Sorry. Just kidding.David WeinbergerMetadata without tearsBut what if we could somehow peek inside our users thought processes to figure out how they view the world? One way to do that is through ethnoclassification  how people classify and categorize the world around them.
He takes del.icio.us and Flickr as examples of "ethnoclassification" (a phrase he tracks back to Susan Leigh Star),. (I am enamored of the branch of ethnoclassification on exhibit at del.icio.us if only because people have started calling it "folksonomy.") He looks at the benefits. Then he addresses the problems, and suggests the paths out of the forest we're making for ourselves.
Jay Fienberg points us also to Jon Udell's article on "collaborative knowledge gardening." I've also been looking at some related issues (e.g., here, here, here, here and here), but Peter has the advantage of knowing what he's talking about.