• Links - Drupal Relationship Module

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:

Ontology Navigation - "defines an ontology, a system of logical terms, for dealing with navigation relationships".

Ontology RDF Module - "defines an ontology, a set of terms used to define an RDF relationship'.

The Relation Manager is built upon ARC - RDF/SPARQL for PHP, a " lightweight, SPARQL-enabled RDF system for mainstream Web projects".