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 means of a plug-in architecture and a Java-based Application Programming Interface (API) for building knowledge-based tools and applications.