Centre national de ressources textuelles et lexicales: type in a word to find out any possible antonym or consult the alphabetical listing of antonymical words.
Synonymes.com: French language and useful links. Enter a word in the dialog box at the top of the page to get its synonyms.Includes links to online translation, conjugation, French idioms, abbreviations, punctuation sites etc. and also links to general and specialized dictionaries.