WordWeb is an English thesaurus and dictionary that works both online and offline. As a matter of fact, further references are taken from the Wikipedia whenever asked to go online.
The free version integrates basic definitions and synonyms for more than 150 000 words. Whenever a word is found, more related words are shown in a separate tab in order to help you in your writing.
Audio pronunciation is given for as little as 5000 words, whereas text pronunciation for 65 000 words. The user interface is rather simple but does its job fast. Users may find useful the existence of a full dictionary and thesaurus for American, British, Canadian, Australian, Indian and Global English.