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:

From the drupal.org post, Semantic Search - Faceted Search and Semantic Web:

From the site:

The Semantic Web Client Library represents the complete Semantic Web as a single RDF graph. The library enables applications to query this global graph using SPARQL- and find(SPO) queries. To answer queries, the library dynamically retrieves information from the Semantic Web by dereferencing HTTP URIs and by following rdfs:seeAlso links. The library is written in Java and is based on the Jena framework.

From the site:

SemWebCentral is an Open Source development web site for the Semantic Web. It was established in January, 2004 to support the Semantic Web community by providing a free, centralized place for Open Source developers to manage Semantic Web software and content development. Another purpose is to provide resources for developers or other interested parties to learn about the Semantic Web and how to begin developing Semantic Web content and software.

Tools for Semantic Web Rules and Ontologies, including Translation, Inferencing, Analysis, and Authoring.

From the site:

The Semantic Web Education and Outreach (SWEO) Interest Group has been established to develop strategies and materials to increase awareness among the Web community of the need and benefit for the Semantic Web, and educate the Web community regarding related solutions and technologies. 

Updated Oct 3 2008: link

The first thing I saw was, "To help in the search for Steve Fossett, please click here ... ". It is not a joke - it is serious.

The next thing was, "Complete simple tasks that people do better than computers. And, get paid for it. Learn more." Yes indeed.

From the site's answer to "What is Mechanical Turk":

From the site:

DBpedia is a community effort to extract structured information from Wikipedia and to make this information available on the Web. DBpedia allows you to ask sophisticated queries against Wikipedia and to link other datasets on the Web to Wikipedia data.

Their demo page for the subject Semantic Web, using DBpedia's Linked Data Interface format.

Freebase is a very powerful environment for data storage and classification. From their site:

Freebase is a uniquely structured database that you can easily search, add to and edit; you can also use the data in it to power your own projects. It’s a data commons in the way that a public square is a land commons—available to anyone to use.

A very concise and straightforward introduction to XML, RDF and the Semantic Web.

Updated Sept 7 2008: added seaBreeze link

Seaside

To get a good feel for Seaside development environment, read the excellent technical introduction by Shaffer Consulting.

Note that the article was written in 2005 and may be obsolete in parts. For example, Seaside now supports several object-oriented databases, including Gemstone.

From the site:

What is SchemaWeb?

SchemaWeb is a directory of RDF schemas expressed in the RDFS, OWL and DAML+OIL schema languages.

SchemaWeb is a place for developers and designers working with RDF. It provides a comprehensive directory of RDF schemas to be browsed and searched by human agents and also an extensive set of web services to be used by software agents that wish to obtain real-time schema information whilst processing RDF data.

Link to the LSDIS (Large Scale Distributed Information Systems) Lab

From the W3.Org site

The Open Data Movement aims at making data freely available to everyone. There are already various interesting open data sources availiable on the Web.

REWERSE is short for "Reasoning on the Web with Rules and Semantics".

From the site: REWERSE is a research "Network of Excellence" (NoE) on "Reasoning on the Web" that is funded by the EU Commission and Switzerland within the "6th Framework Programme" (FP6), Information Society Technologies (IST), Priority 2 under the project reference number 506779. REWERSE addresses the IST strategic action line "Semantic-based knowledge systems".