The Protege Project offers WebProtege and Protege Desktop, which are free and open source ontology editing applications.
WebProtégé is an ontology development environment for the Web that makes it easy to create, upload, modify, and share ontologies for collaborative viewing and editing. WebProtégé fully supports the latest OWL 2 Web Ontology Language. The highly configurable user interface creates the perfect environment for beginners and experts alike. Collaboration features abound, including sharing and permissions, threaded notes and discussions, watches and email notifications. RDF/XML, Turtle, OWL/XML, OBO, and other formats available for ontology upload and download.
Protégé Desktop is a feature rich ontology editing environment with full support for the OWL 2 Web Ontology Language, and direct in-memory connections to description logic reasoners like HermiT and Pellet. Protégé Desktop supports creation and editing of one or more ontologies in a single workspace via a completely customizable user interface. Visualization tools allow for interactive navigation of ontology relationships. Advanced explanation support aids in tracking down inconsistencies. Refactor operations available including ontology merging, moving axioms between ontologies, rename of multiple entities, and more.