WordWeb is a free, convenient and powerful dictionary and thesaurus for English. It can look up words from inside almost any program. It works off-line, but can also look up words in web references such as the Wikipedia encyclopedia. Features of the free version include: Definitions and synonyms, Proper nouns, Related words, Pronunciations, 150 000 root words, 120 000 synonym sets.
WordWeb not only finds synonyms of words, but also a whole host of other words related in some way to the searched word. When you first look up a word WordWeb displays the synonyms in the lower half of the window. The other types of related word available are shown in the set of tabs at the top of the list box. Just click on one of these to show the related words of that type.
Many words have more than one meaning, so when you lookup a word there many be more than one definition displayed. To show just the synonyms specific to a particular sense, just click on the sense number.
English pronunciation varies considerably across the English speaking world. WordWeb provides a rough guide, concentrating on the standard American and RP (BBC English) varieties that are widely comprehended. Besides visual representation through letters, WordWeb will let you also hear how a word sounds, by offering actual audio pronunciation.
(Check also this free Wikipedia Sidebar Gadget with multilanguage support)