Del.icio.us is one example of how such folksonomies can be really useful, and here's another (and note the groovy .museum domain - the first time I've seen this):
"Steve” is a collaborative research project exploring the potential for user-generated descriptions of the subjects of works of art to improve access to museum collections and encourage engagement with cultural content. We are a group of volunteers, primarily from art museums, who share a common interest in improving access to our collections. We are concerned about barriers to public access to online museum information. Participation in steve is open to anyone with a contribution to make to developing our collective knowledge, whether they formally represent a museum or not.
Very cool - both in terms of adding metadata to objects, and as far as getting the public involved with art. Indeed, this idea should really be extended to everything - imagine a database of public places that people could tag.
Great idea, then, but why "Steve"?