Altova SemanticWorks: "Altova SemanticWorks™ 2006 is the ground-breaking visual RDF/OWL editor from the creators of XMLSpy. Visually design Semantic Web instance documents, vocabularies, and ontologies then output them in either RDF/XML or N-triples formats. SemanticWorks™ 2006 makes the job easy with tabs for instances, properties, classes, etc., context-sensitive entry helpers, and automatic format checking. It is the sensible way to put the Semantic Web to work for you."
Also check out Altova's Professional XML Suite Features matrix