When Clay Shirky posted his now famous "Ontologies are overrated" paper, (see also here) a relatively new conversation started. Slipping in the sidelines, however, massive creativity continues. For instance, look at this page at del.icio.us where a small ontology of file types is used to refine the del.icio.us tags.
A comment to that post points to here, a page which enumerates things going on either for, around, or inspired by del.icio.us. One such link points to sid.vicio.us where a list of OWL ontologies exists, one of which is liberal.owl. rdf:about and rdf:resource attributes are links into del.icio.us content.