Updated Sept 16 2008: Note that this is not a complete list of Java workflow applications, just the ones that I have found and interest me.

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.

Not sure if WordNet qualifies as the "Semantic Web", but it's definitely semantic whatever the format may be. From the site ( paraprased )

WordNet® is a lexical database containing nouns, verbs, adjectives and adverbs. They aregrouped into "synsets" ( sets of cognitive synonyms ) each expressing a distinct concept. Conceptual-semantic and lexical relations build links between synsets. A browser navigates through the network of meaningfully related words and concepts.

Oct 5 2009: PyKE 1.0.2 download, seems to be fairly active
Updated Jan 26 2009: new SimPy 2.0 release, Twisted framework, more links
Updated Oct 29 2008: added Narval

An important and growing segment of the rule engine world ... Python runs fast, a necessary requirement for all large rule-based applications.

More pattern matching and structural parsing than symbolic processing in a classical sense of an inference engine.

Updated Oct 3 2008:

C++ engines are often neglected for web-based inferencing applications, but compilation is possible on non-shared hosting ...

Of the 702 Semantic Web tools listed in Mike Bergman's masterlist, 332 are implmented in Java .... and Java now being a fully open source and open licence language ...

Stray link I'll find a home for: http://www.daml.org/tools/

SWCL - Semantic Web Client Library:

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".

An interesting development in the evolution of Linux, almost a new species of Linux as it were.

andLind is a native port of the Linux kernel and Ubuntu to run as an application under Windows 2000. The two versions support either XFCE or KDE. With a few restrcitions.

Feb 3 2009: reflections on the power of modern marketing in selecting rule engines.

Sun appears to be serious about open sourcing the Java Development Kit and Virtual Machine, so it is time to consider Java as an environment for implementing Open Source rule and inference engines.

As an interpreted language, the performance and resource consumption of client and server-side solutions will probably be a critical design factor, particularly as Java-based rule engines tend to aspire to enterprise-level applications.

From the site:

FDSE is an easy-to-install search engine for local and remote sites. It returns fast, accurate results from a template-driven architecture.

Some interesting possibilities that may lead to a better RETE algoritm. There are some good article suggesting directions for investigation but references on the web are still a bit thin.

There is an interesting relationship between the RETE algoritnm and an emerging field called Join Calculus. The RETE algorithm has always suffered from a sense of being ad-hoc, it works therefore it is, without any real explanation for why it works so well in so many situations. Join Calculus may be able to provide a more general grounding for building rule engines than RETE, or at least give RETE a stonger theoretical foundation.

The Semantic Web held pride of place at the AAAI 2008 Spring Symposium Series.