From the website: "OpenCyc is the open source version of the Cyc technology, the world's largest and most complete general knowledge base and commonsense reasoning engine."
I'm not sure about their claims, however, the project looks very interesting.
OpenCyc can be used as the basis of a wide variety of intelligent applications such as:
rapid development of an ontology in a vertical area
email prioritizing, routing, summarization, and annotating to name just a few.
Release 1.0 of OpenCyc will include:
6,000 concepts: an upper ontology for all of human consensus reality.
60,000 assertions about the 6,000 concepts, interrelating them, constraining them, in effect (partially) defining them.
A compiled version of the Cyc Inference Engine and the Cyc Knowledge Base Browser.
A suite of "RKF" tools for rapidly extracting knowledge from a domain expert (e.g., a physician or oil drilling specialist), tools which operate by carrying on a clarification dialogue with that individual; hence: tools for answering questions via English dialogue.
Documentation and self-paced learning materials to help users achieve a basic- to intermediate-level understanding of the issues of knowledge representation and application development using Cyc.
A specification of CycL, the language in which Cyc (and hence OpenCyc) is written. There are CycL-to-Lisp, CycL-to-C, etc. translators.
A specification of the Cyc API, by calling which a programmer can build an OpenCyc application with very little familiarity with CycL or with the OpenCyc KB.
The ability to import and export CycML files.
A few sample programs that demonstrate use of the Cyc API for application development.