OpenCyc is the open source version of the Cyc(r) technology, the world's largest and most complete general knowledge base and commonsense reasoning engine. OpenCyc can be used as the basis for a wide variety of intelligent applications.
Today release 0.6 is available for linux on i386 platforms. Discuss it on irc.openprojects.net / #opencyc.
Documentation, KB tours and tutorial materials at http://www.opencyc.org/doc