Another resource for those interesting in collaborative tagging. See the site for additional papers:
FaceTag is a working prototype of a semantic collaborative tagging tool conceived for bookmarking information architecture resources. It aims to show how the flat keywords space of user-generated tags can be effectively mixed with a richer faceted classification scheme to improve the system information architecture.
Facets constitute an adaptive classification system capable to represent both a knowledge in movement (like that of collaborative environments) and several mental models at the same time. The blend of tags and facets is able to empower the information scent and berrypicking capabilities of the system.