WordWeb provides users with word and common phrase definitions plus a pretty useful list of synonyms and antonyms. The latest versions also incorporate 5000 audio pronunciations. It integrates with literally anything which contains text, both offline and online: Simply press the ?Ctrl? key and then right click on the word (or phrase).
Here is a typical result from a Word document:

And from a web page:

So useful to check meanings and correct word usage; especially useful for writers to quickly look up synonyms so they are not repeating the same word over and over. Highly recommended!!
Features:
*Definitions and synonyms
*Proper nouns
*Related words
*5000 audio pronunciations
*150 000 root words
*220 000 word senses
*Fixed web reference tabs
Technical:
Download Size: 17.84MB
Installed size (Windows 7): 28.6MB
Supported operating systems: Windows 2000/XP/Vista/Windows 7
Supported languages: American, British, Canadian, Australian, Indian, and global English.