OWL

OWL is the Web Ontology Language

Some excellent resources for information about and/or tools for OWL are:
 * Co-ode - see in particular their tutorials
 * The open source Pellet reasoner and OwlSight ontology browser supported by Clark and Parsia
 * Manchester University's OWL resources
 * TopBraid Composer, a commercial ontology editor from TopQuadrant
 * Uli Sattler's list of description logic reasoners
 * The Protege project. Both the open source version 3 and version 4 support OWL.

Note, Science Commons's own Alan Ruttenberg is one the chairs of the working group specifying the new OWL 2.