OWL2XMI is a java project to facilitate the creation of classes UML diagrams. It generates a XMI file from one ontology, this XMI file can be imported using UML tools such as: StarUML, ArgoUML, among others.
Now! It is available the online version: http://swap.uib.es:8080/owl2xmi/
In this project is not our intention explain the trouble of convert OWL ontologies into UML representation.
Since, UML does not support ontology features, such as: subproperties; equivalent, transitive,..., reflexive properties, etc.
You can obtain more information:
It is Free software distributed under the GNU General Public License (GPL)