OpenCyc is the open source version of Cyc and is a product of Cycorp. It is, in reality, a subset of Cyc and only its knowledge base is open source. An inference engine is available for free, but it is closed source.A number of public OpenCyc servers are available on the Web.OpenCyc can be used for a variety of purposes. These including database integration and construction of ontologies for expert systems.
My personal experience is that OpenCyc can be convenient for representing knowledge. However, it is very difficult to work with as far as inferencing is concerned. First-time users should be aware that it can only handle Horn clauses and perform inferences using Modus Ponens and Modus Tollens.