Windows/Linux: This app is for all you wordsmiths and word buffs. Artha is a free thesaurus and dictionary with extensive features, including hotkey lookup, notifications, regular expression-based search and offline use.
Artha is based on WordNet, the large lexical database of English, in which words are linked semantically.
Depending on the term you lookup in Artha, the program will offer not only your typical part of speech, meaning and synonyms, but other useful details, such as derivatives, attributes, domain terms, parts of, and other words that are similar.
The hotkey lookup makes finding meanings very easy: press the default combination Ctrl+Alt+W after selecting a word to look it up in Artha.
Notifications shows you word meanings in a balloon tip, so you can continue working uninterrupted.
Finally, you can also look up words using regular expressions. So if you don't know the exact spelling, you can type in something like "sp*g" to get suggestions.
Artha has been out for Linux for some time and more recently released for Windows. Upcoming versions will add pronunciation. Unlike other dictionaries and thesauruses that do their lookups by querying web servers, Artha can be used offline.