I spent the first week of the new year (it was a very quite week) playing with ontologies. Major cudos to the group at Stanford that is responsible for Protege. Protege is an ontology editor, opensource and free. I’ve installed and played with the tool and it is quite decent, but the Protege-OWL tutorial that accompanies it is simply top notch! The tutorial is a valuble read even if you don’t have the tool, though being a hands-on person I found it even better with.