Suggested Upper Merged Ontology
- A free, formal ontology in first order logic with domain ontologies totalling 20,000 concepts and 60,000 axioms, with mappings to all of WordNet.
- http://www.ontologyportal.org

OpenCyc
- OpenCyc is the open source version of the Cyc technology, the world's largest and most complete general knowledge base and commonsense reasoning engine.
- http://www.cyc.com/cyc/opencyc/overview

Semantic Web for Earth and Environmental Terminology
- Several ontologies published by the Jet Propulsion Laboratory in OWL format. Includes Space, Units, Earth Realms, Phenomena, Physical Properties, Substances and Units.
- http://sweet.jpl.nasa.gov/ontology

AKT Reference Ontologies
- Various ontologies developed in the framework of the Advanced Knowledge Technologies project. Includes a portal ontology.
- http://www.aktors.org/publications/ontology/

Business Process Management Ontology
- BPMO is an integrated information model including business process design, project management, requirements management, and business performance management.
- http://www.bpiresearch.com/Resources/RE_OSSOnt/re_ossont.htm

Descriptive Ontology for Linguistic and Cognitive Engineering
- DOLCE is the first module of the WonderWeb Foundational Ontologies Library (WFOL).
- http://www.loa-cnr.it/DOLCE.html

National Cancer Institute Thesaurus
- An OWL translation of the NCI Thesaurus, available for download (32 Mb) or online viewing.
- http://www.mindswap.org/2003/CancerOntology/

Open Biological Ontologies
- Provides access to controlled vocabularies for shared use across different domains, like MESH and Gene Ontology.
- http://obo.sourceforge.net

Ontology for Geographic Information and Metadata
- An OWL publication of the ISO 19115 standard, from Drexel University.
- http://loki.cae.drexel.edu/~wbs/ontology

OWL Ontology Library
- A selection of OWL ontologies usable in Protégé.
- http://protege.stanford.edu/plugins/owl/owl-library/index.html

ThoughtTreasure
- A web-accessible ontology system with terms in English and French. A downloadable version is also available.
- http://www.signiform.com/tt/htm/tt.htm

The Enterprise Ontology
- Collection of terms and definitions relevant to business enterprises. Developed in the Enterprise Project by the Artificial Intelligence Applications Institute at the University of Edinburgh.
- http://www.aiai.ed.ac.uk/project/enterprise/enterprise/ontology.html

DAML Ontology Library
- An index of more than 200 publicly available ontologies expressed in DAML.
- http://www.daml.org/ontologies/