Updated Oct 3 2008:

Libraries and a toolkit for developing semantic web applications using MicroSoft .NET technology. Not open source of course, but their developer documentation in interesting.

Updated Sept 24 2008: added features and SWCL link

Updated: Sept 24 2007

It is a browser plugin, which introduces browser dependencies, but the project is Open Source and seems to be active and well-supported.

From the site:

Mozilla RDF was originally used to support the Aurora/Sidebar user interface and SmartBrowsing metadata services. It's main use in Mozilla now is as a common data model and API for use in XUL-based applications.

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:

From the project page:

Node linking and Metadata

The Relationship Manager module is intended to allow free-form links between nodes, recording not just that one page relates to another, but HOW it relates. ... The core module is made available to be extended by other modules that wish to make use of the generalized relationship management functions. Several examples of simple modules that leverage this power are provided as optional plugins.

The major plugins are:

Updated Sept 19 2008:

From the article "Using Prolog as the fundament for applications on the semantic web":

Oct 26 2009: there may be some activity for running Glade XML with XUL, new link for the Webglade Javascript library.
March 29 2009: moved server-side Javascipt, and added yet more links.
March 27 2009: added Jelix and Jaxer, more links.

See related content about Firefox and server-side Javascript.

Feb 17 2009: cheap servlet hosting ?
Feb 5 2009: more links to applet demos

A few ill-formed thoughts so far ...

Java is The Enterprise Language. Most Java applications are not applications so much as pieces in the jigsaw puzzle of the enterprise integration architecture. In short, they are big, complex and expensive.

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.

A strong candidate for a Smalltalk RDF/OWL inference engine is the Smalltalk Open Unification Language (SOUL).