Visuwords – Online Graphical Dictionary
Visuwords not only look ups the definitions of words but also their associations with other words and concepts. It is based on Princeton University’s WordNet. Despite that Visuword doesn’t provide the audio pronunciation for each word, it is just like another similar but commercial product, Visual Thesaurus (also based on WordNet). But it can save you annual subscription fee $19.95.
If you want a desktop version, you can try George Washington University’s JWord. The interface is not so fancy, but enough for a thesaurus dictionary.