Loom is now open source (a BSD-like license). "Loom is a language and environment for constructing intelligent applications. The heart of Loom is a knowledge representation system that is used to provide deductive support for the declarative portion of the Loom language. Declarative knowledge in Loom consists of definitions, rules, facts, and default rules. A deductive engine called a classifier utilizes forward-chaining, semantic unification and object-oriented truth maintainance technologies in order to compile the declarative knowledge into a network designed to efficiently support on-line deductive query processing."
An article theServerSide.com introduces "...the JSR-94 Rules Engine API and an Open Source product called Drools, the forerunner implementation of this up-and-coming technology...it can scale to incorporate and execute hundreds of thousands of rules in a manner which is an order of magnitude more efficient then the next best algorithm." Drools Homepage.