The last release 2.0 was made available almost three years ago, so I was very happy to see that version 4 is available.
OpenCyc is a knowledge base and ontology containing about 270K terms and 2 million triples. There are now many more links to outside (or OpenCyc) knowledge resources like DBPedia, UMBEL, Wordnet, etc. Check out the new features.
I downloaded the Linux version (requires Java 6, 3 GB memory, and is only for 64 bit OSs and Java). I had no problems at all running it on 64bit Ubuntu and OS X Mountain Lion (developer preview 4) - it only took a few minutes to install and run. Just follow the instructions in the README.txt file and try the web interface at http://localhost:3602/cgi-bin/cg?cb-start Nice to see this the free version of Cyc getting support and development efforts.
I would be very interested in hearing from people who have done projects with either OpenCyc or the OpenCyc OWL/RDF data.
9-9-2012 update: Alyona Medelyan provided a link for downloading data to extend Cyc with terms from Wikipedia.