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.