Following Joi Ito, I am going to quote David Weinberger’s post in its entirety. I’ve really become interested in the way tagged meta data works (see this), and this post contains lots of useful links:
But 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.
UPDATE: BoingBoing has a post about how you can now subscribe to RSS feeds of all the flickr photos you’ve commented on!