Updated Sept 19 2008: added Ruby/EventMachine links
Updated Sept 19 2008: more links for Ruleby

Not a major category among the many rule engines implemented in Java and other languages. In this case, I've pushed the definition of 'engine' to the limits so I can include some of the more solid and interesting implementations.

Updated Sept 24 2008: added features and SWCL link

Updated Sept 28 2008: fixups and links

From the CakePHP project site at ohloh.net:

Cake is a rapid development framework for PHP which uses commonly known design patterns like ActiveRecord, Association Data Mapping, Front Controller and MVC. Our primary goal is to provide a structured framework that enables PHP users at all levels to rapidly develop robust web applications, without any loss to flexibility.

March 19 2009: Pharo will be going beta with version 1.0 !!! I thought they said it would take three years ... glad it didn't :-).

Updated Sept 16 2008

Semantic web search engines ? Social bookmarking run mad ? We shall see. The new search technology is still early in the S-curve and more useful for pop culture subjects than for rule based systems.

From the WikiaSearch project page:

Wikia's search engine concept is that of trusted user feedback from a community of users acting together in an open, transparent, public way.

Updated Oct 2 2008: The Mission

There is a decided dearth of good PHP open source search engine solutions, but Sphider seems to be the leading candidate these days - "a lightweight web spider and search engine written in PHP, using MySQL as its back end database".

March 19 2009: Squeakland-EToys updated, see the EToys download page.
Updated July 10 2008: Pharo - new, leaner fork of Squeak. See related link.

From Wikipedia on Squeak Smalltalk:

It's new, it's edgy and in your muzzle, it's pawing its way to the top of distrowatch.com download rankings and nosing out the competition. The future King of the Multimedia Kennel.

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:

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"

Potential RDF use cases for Drupal - an excellent article about RDF in Drupal 6 - 7.

The Drupal RDF module uses the recently-rewritten ARC2 RDF Library if available but apparently does not require it.

Feb 20 2009: link for building a KDE .sfs
Feb 10 2009: devxx.sfs. for hard disk install

A full collection of Puppy links. They can be hard to track down individually.

Notes: Hard-Puppy page about updating o disk installation with devxx,sfs:

Also a link to CCK-related modules.