Shirky has a great article, Ontology is Overrated—Categories, Links, and Tags which describes some of the advantages of tagging systems versus a typical hierarchical ontology. I love the Dewey Decimal System example categorization for religions of the world…

Dewey, 200: Religion
210 Natural theology
220 Bible
230 Christian theology
240 Christian moral & devotional theology
250 Christian orders & local church
260 Christian social theology
270 Christian church history
280 Christian sects & denominations
290 Other religions

No bias there, eh?

And a couple on the del.icio.us/flickr idea of tagging…

“…you can see there’s a tag “to_read”. A professional cataloguer would look at this tag in horror—“This is context-dependent and temporary.” Well, so was the category “East Germany.”

That’s funny. And this…

“The addition of a few simple labels hardly seems so momentous, but the surprise here, as so often with the Web, is the surprise of simplicity. Tags are important mainly for what they leave out. By forgoing formal classification, tags enable a huge amount of user-produced organizational value, at vanishingly small cost.”