A top-down, human-driven attempt to amass an inference database of common sense relationships. It's been going 20 years now, started by DougLenat.
In recent years, the Cyc Project has been replaced by a commercial company Cycorp, to move the knowledge base into its next frontier. It's most recently announced that it will be releasing the source to its knowledgebase at http://www.opencyc.org. In fact a beta version has now been released (April 2002) although it is greatly scaled down from the full commercial version.
Competing directly with it are other Knowledge Representation projects like the MindPixel?, SUMO and OpenMind? projects.