Updated Sept 22 2008:

A few AI links won't hurt my prospects for future employment ... will they ?

People ask the way to Cold Mountain.
Cold Mountain ? There's no road that goes through.

Han-Shan - "Cold Mountain"

Drools is probably the leading Open Source Java rule engine and rule management environment. It is the community version of commercial JBoss Rules. The whole operation os very solid - the Drools/JBoss Rules User Guide is among the best of any business rules product, either open source or commercial.

The following is a heavily paraphrased version of the three main points on Drools site:

Updated: Sept 24 2008: more links, RAP

A very thin category at the moment, but the new Drupal Rules project may help to fill it out.

The RAP project has a simple forward and backward chaining inference engine using networks of relationships rather than condition/action statements like a true rule engine.

Big, non-open, usually implemented in Java or C++.

Updated: June 20 2008

Galaxia is an activity-based workflow application, where the processes and the workflows are made of dynamic activities that must be completed to finish a task. Contrast this with a procedural system where each step in the process must be completed in order, and the power of this application becomes clear.

There are critical differences between the kind of inferencing done by Semantic Web applications and the inferencing done by rule engines. A simple explanation might be that SemWeb apps are more knowledge-intensive and rule engines are more process-intensive, particularly in a business context. Potential, this might change over time, but of today, I would say the following applies.

Update: Jan 16 2009 - Current development release ( rules-6.x-1.x-dev ).
Update: Sept 29 2008 - Current production release ( 6.x-1.0-beta3 )

Wolfgang Ziegler ( a.k.a. fago ) has started a project derived from the workflow-ng module to create a Drupal rules nodule implementing inference engine functionality, tightly coupled to the Workflow-NG module. One presumes that the rules engine will implement forward/backward chaining and be pluggable into the workflow engine.

The game's a foot.

From the Workflow-NG project page, a list of Drupal 5 modules that integrate with Workflow-NG ( other than Workflow ):

It's getting serious. Things are moving resolutely forward toward Drupal 6 and 7.

The seems to be some confusion about the difference between 'rule engines' and 'semantic web' applications.  See related link to "Semantic Web Inferencing versus Rule Engines"

Update 3/26/2008 - ARC completely reimplemented and released as ARC2.

Update 7/25/2008 - added Triplify 0.3 section.

From the ARC site:

ARC is a lightweight, SPARQL-enabled RDF system for mainstream Web projects. It is written in PHP and has been optimized for shared (or otherwise performance- or privilege-limited) Web environments.

ARC facilitates the integration of RDF and SPARQL functionality in PHP/MySQL-driven frameworks.

Updated Sept 25 2008: new link This page describes the taxonomy of semantic web topics adopted by the European projects Knowledge Web and REWERSE

OntoGrid provides a technical infrastructure for the rapid prototyping and development of knowledge-intensive services for the Semantic Grid.

Opera has several browser plugins and lively projects to build support for the Semantic Web.

One of most interesting plugins is the Tabulator RDF Browser Plugin. It's one of the few RDF tools that actually works.

The Tabulator Developers Notes ( and source code ) at MIT's DIG-CSAIL.

The main link is to Oracle - Semantic Technologies Center. so far .... From the site:

Semantic Technologies are designed to extend the capabilities of information on the Web and enterprise systems to be networked in meaningful ways. ... Oracle Spatial 11g introduces the industry's first open, scalable, secure and reliable RDF management platform. Based on a graph data model, RDF triples are persisted, indexed and queried, similar to other object-relational data types.

Updated: Oct 4 2007

The most complete and developed example of the Semantic Web to date is probably Semantic MediaWiki at ontoworld.org. Note that its vision is in the Wiki on Steriods school of thought, which is one of several overlapping but distinct visions. From the site: