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.