Received an email from Adam Pease on my outdated links so I updated them in the resources section and the UBL ontology project (no longer active). SUMO is an open source upper ontology using the Knowledge Interchange Format (KIF) syntax. All the terms in SUMO are fully axiomatized. Additionally, the terms have been mapped to Wordnet synsets. While I have not done a rigorous analysis of all the available upper ontologies, I have known Adam for awhile and am certain that he knows this domain and has made real strides in the area of knowledge representation. I highly recommend you examine SUMO and its use for semantic integration, reasoning and categorization.
My Corner of the Web