Since I read a Discover magazine article about this back in the mid-80s, I've been interested to see where this technology would end up.

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." The goal of the project is to give computers the ability to reason about data, and to possibly converse directly with humans in plain English.

The file that stores OpenCyc's "world" is just over 500MB.

License: Apache 2
Home page: opencyc.org
IRC: #opencyc
Video: on YouTube

I'm currently interfacing to the OpenCyc via it's Java API, but using Clojure on top of Java in order to make the experimentation much easier. The normal way to interface with OpenCyc is via the OpenCyc Browser-- a closed-source web application that runs locally.