Resource Description Framework (RDF)
The Resource Description Framework project is an ensemble of modules providing comprehensive RDF functionality and interoperability. In 2009, Stéphane "scor" Corlosquet and others published Produce and Consume Linked Data with Drupal! which presents the approach for using Drupal (or any other CMS) as a Linked Data producer and consumer platform. This paper presented what was possible at the time in Drupal 6. Some of this work was integrated into Drupal 7 core. Active development has now moved on to building contributed modules for Drupal 7, leveraging Drupal's core RDFa and RDF mapping API.
RDF is a W3C standard for modeling and sharing distributed knowledge based on a decentralized open-world assumption. Any knowledge about anything can be decomposed into triples (3-tuples) consisting of subject, predicate, and object; essentially, RDF is the lowest common denominator for exchanging data between systems.
See also the Semantic Web group on groups.drupal.org.