Good introduction to the concepts of folksonomy and tagging, though if you are familiar with the topic you won't learn a lot that's new. Especially useful is the list of advantages and disadvantages of folksonomies. The author concludes, correctly, "The advantages to top-down hierarchical taxonomies for library collections are without question. For cataloging the Web, however, they just aren't feasible." A lot of people working on traditional (structured and formal) semantic web applications will learn this the hard way. Via EDUCAUSE. For a related read, be sure to check this item from David Davies on the incompatible formats used by different entities (such as Flickr and del.icio.us) to denote tags.