Facile à dire!
This is the French forum at Wordreference.com. You can do a dictionary and thread search not only for words but for expressions that are difficult to find in most dictionaries, including slang expressions. Under "Resources," you'll find links to discipline-specific online dictionaries, and you can ask questions in the French-English vocabulary subforum.
This thread at Wordreference French forum gives links to dictionaries, sites that can help with verb conjugations, and sites for general help with French grammar.
The Larousse site offers a general French-English English-French dictionary, a thesaurus, a dictionary of expressions, a dictionary of quotations, and verb conjugation help.
A French thesaurus.
Lexilogos has links to numerous French (monolingual) dictionaries, including dictionaries of Old French and French language from the 16th-20th centuries.
Click on ARTFL. The ARTFL main database has thousands of searchable French works, primarily literary works. You can do searches for specific words or expressions. This function can be especially useful if you are writing a paper and you want to look for themes, or if you want to avoid looking through an entire book to find a specific quotation that you're looking for.
Vis-à-vis Online Learning Center: www.mhhe.com/visavis5
Embassy of France in Washington: http://www.info-france-usa.org/
Le Devoir: http://www.ledevoir.com/
Le Monde: http://www.lemonde.fr/