Good article in the Guardian about Tagging. Except this statement:
But, extending an idea developed by SocialText for its wikis, del.icio.us also encourages users to "tag" bookmarks...
This implies that Socialtext invented tagging, which is, uh, categorically false.
Socialtext Categories share similar properties to tags, and was implemented two years ago, but takes a different approach. Its a flat namespace, anyone can add/delete/modify a category with a simple interface, very wiki-like. We also added rel tags to Categories to support Technorati's tag search. You can play with it here.
On the other hand, we did invent metadata ;-)