Ontology construction

Protege

A Java-based program to construct OWL and RDF ontologies.

Open source: yes
Link: http://protege.stanford.edu/

TopBraid Composer

Very powerfull application for constructing OWL and RDF ontologies. Implemented as a Eclipse plug-in, which gives you the freedom to develop your own plugins easily.

Open source: yes (TBC free), there are also commercial versions availiable (TBC standard & TBC Maestro)
Link: http://www.topquadrant.com/products/TB_Composer.html

Wonderweb OWL Validator

A tool to check whether the OWL you submit has a valid OWL Lite, OWL DL or OWL Full syntax.

Link: http://www.mygrid.org.uk/OWL/Validator