Protégé is a free, open source ontology editor that hides the complexity of underlying RDF and OWL syntax through a user-friendly graphical interface. The Protégé effort is supported by the National Library of Medicine with help from Stanford University, which offers courses and sponsors annual conferences on Protégé.
Using Protégé’s interface, it’s possible to define domain entities and specify relationships. The strength of Protégé is its ability to export data in a variety of formats, including RDF, OWL, and even XML Schema. The Protégé web site hosts a wiki for ontologies where users share their experiences building ontologies. The site also includes numerous sample ontologies. With interest growing in the semantic web, ontology building has been quite popular in recent years, and you may find that an ontology already exists for your application area.