Protégé is a free, open source ontology editor and knowledge-base framework. The Protégé 4 platform supports modeling ontologies via OWL using the OWL API. Results from Protégé ontologies can be exported into a variety of formats including RDF(S), OWL, and XML Schema.
Protégé is based on Java, is extensible, and provides a plug-and-play environment that makes it a flexible base for rapid prototyping and application development.
Protégé may be downloaded from here.