Protege
Protege is a free, open-source platform that provides a growing user community with a suite of tools to construct domain models and knowledge-based applications with ontologies. At its core, Protege implements a rich set of knowledge-modeling structures and actions that support the creation, visualization, and manipulation of ontologies in various representation formats. Protege can be customized to provide domain-friendly support for creating knowledge models and entering data. Further, Protege can be extended by way of a plug-in architecture and a Java-based Application Programming Interface (API) for building knowledge-based tools and applications.
The Protege platform supports two main ways of modeling ontologies via the Protege-Frames and Protege-OWL editors.
Protege Overview: http://protege.stanford.edu/overview/
Protege-Frames Overview: http://protege.stanford.edu/overview/protege-frames.html
Protege-OWL Overview: http://protege.stanford.edu/overview/protege-owl.html
Protege User Documentation: http://protege.stanford.edu/doc/users.html
Protege Developer Documentation: http://protege.stanford.edu/doc/dev.html
