Cyc
From semanticweb.org
Cyc is an artificial intelligence project that attempts to assemble a comprehensive ontology and database of everyday common sense knowledge, with the goal of enabling AI applications to perform human-like reasoning.
One stated goal is that of providing a completely free and unrestricted semantic vocabulary for use in the Semantic Web. The OpenCyc taxonomy is available in Owl on the OpenCyc web site.
The Cyc Foundation works to expand the open source ontology and make semantic links in wikipedia from Cyc.