OWLAx
A Protégé plugin for visual ontology development through diagramming.
OWLAx is a Protégé plugin that supports ontology axiomatization through diagramming, letting ontology engineers sketch class/property structures visually and generate OWL axioms automatically. It has become a widely used tool in the Semantic Web community.
Open source: github.com/md-k-sarker/owlax