How to visualize an ontology

Finding a tool to visualize an ontology's structure is what this challenge is all about. It should be as simple and intuitive as possible, and, if possible, free of charge. It should be able to handle at least one of the standard ontology standards (e.g. OWL).
1 answer

Protege's plug-in OWL Viz

Protege, the ontology editor and knowledge acquisition system, has a plug-in which is used to visualize ontologies. Since some time now, the plug-in is installed with the Protege system. There is a problem though. The plug-in, OWL Viz is dependent on the graph visualization software Graphviz, which needs to be installed separately (can be downloaded here). If it is not installed to it's standard location, then Protege need to be pointed to it. That is done on Windows as following:

1. File->Preferences
2. Click on the Tab OWL Viz
3. Under General Options set the Dot Application Path to dot.exe (in Graphviz's bin folder).