TagOnt is an OWL ontology describing the concept of tagging (folksonomy), i.e. it is basically a formal description of tags, resources and their relations. You can find all source files in the SVN.

The screenshot below shows the big picture, please refer to the paper for more detailed information as well as the rationale for the design decisions.

Besides the OWL ontology itself, there is also some sample code for accessing the ontology using the Jena toolkit as well as an importer to fill the ontology with sample code from del.icio.us.