Linguee

http://www.linguee.fr/
This french-english dictionnary is very simple to use.
Interface language: French

Word Reference

http://www.wordreference.com/
This dictionnary is one of the best known worldwide. It provides translators and dictionnaries in a lot of languages.
Interface language: English, French, Chinese, Spanish, Italian, Japanese, Portuguese, Russian, German, Arabic, ...

Dictionnaire TV5

http://dictionnaire.tv5.org/dictionnaire
This very handy dictionnary can help you find definitions, synonyms, conjugation, and translations into english.
Interface language: French

Le Dictionnaire Visuel

http://www.infovisual.info/index_fr.html
This very unsual dictionnary uses pictures to complete its definitions and therefore is a very interesting material for children and beginners.
Interface language: French, English, Spanish

Encyclopédie Larousse

http://www.larousse.fr/encyclopedie
You want more than a simple definition? Here is an online encyclopedia that will help you understand complex concepts and ideas.
Interface language: French, English, German, Spanish, Italian

Reverso

http://dictionnaire.reverso.net/francais-definition/
This dictionnary allow you to search for synonym, definitions, conjugation, quotes containing certain words or translations in english.
Interface language: English, French, German, Spanish, Italian

Le Trésor de la Langue Française informatisé (TLFi)

http://atilf.atilf.fr/

Le Tésor de la langue française offers an outstanding dictionnary with phonetic recognition features available (write a word like you hear it and you will find its orthograph and definitions).

Interface language: French