Link - Commercial Inference Engines
Updated Sept 8 2008: added BizTalk links ( after much agonized debate with myself )
Generally identical to "enterprise" business rule engines, usually implemented in Java or C++. This category excludes many pattern or search engines geared toward business intelligence and data mining.
Note the Microsoft BizTalk links. While BizTalk is and will remain resolutely closed licence and closed source ( and thereby contrary to the goals of this site ), the documentation for BizTalk is really excellent, perhaps the best in the rule engine industry. It's a "must read" if not a "must buy and use".