Parley is a vocabulary trainer. It helps you to memorize your vocabulary, for example when you are trying to learn a foreign language.
Rapunga Tere
Quick Lookup is a simple GTK dictionary application powered by Wiktionary™.
Artha
Artha is a free cross-platform English thesaurus that works completely off-line and is based on WordNet
Dictionary
This program allows you to search different kinds of dictionary services for words or phrases and shows you the result.