Wiktionary & Wikisaurus

Wiktionary,the free dictionary

2,170,516 entries with English definitionsfrom over 400 languages

http://en.wiktionary.org/wiki/Wiktionary:Main_Page

Wiktionary: Wikisaurus

This is the main project page of Wikisaurus—a Wiktionary subproject and a wiki namespace aiming at creating a thesaurus—a dictionary of synonyms, antonyms and further semantically related terms such as hyponyms, hypernyms, meronyms and holonyms.

http://en.wiktionary.org/wiki/Wikisaurus