The CYC Commonsense Knowledge Base @ UIUC (Research Edition)
The CYC Commonsense KB, developed by Cycorp Inc., is probably the most extensive formal collection of commonsense knowledge currently available. Click here for access to the research edition of the KB hosted at UIUC.
OpenCyc is a free version of the Cyc KB distributed under the Creative Commons License. Here is a first order version of the OpenCyc KB Version 0.9 in the TPTP format, created by Deepak Ramachandran, Keith Goolsbey and Pace Reagan. Before downloading, please read the license first.