Bilingual dictionaries

Bilingual dictionaries are an invaluable aid when you are learning a language whatever your level.  A small pocket bilingual dictionary is useful for quick translation of a word and to carry around, but a comprehensive dictionary is needed to study a language adequately.  Comprehensive dictionaries are more than translations of words; they teach different meanings, idioms, syntactic constructions, grammar and more: they are real facilitators of expression in the foreign language.
The Collins Robert French Dictionary places emphasis on current French and English, both written and spoken. The basic vocabulary in each language is treated exhaustively with a wealth of examples illustrating usage, including idioms, set phrases and syntactic constructions.  Most useful is a 72-page section at the back of the book entitled "Language in use" which is a grammar of communication in French and English arranged thematically to facilitate self-expression in the foreign language (for example: to give advice, how to telephone, letter writing et.).
The Oxford Hachette French Dictionary contains over 900,000 words, phrases, and translations of French as it is used today.  It contains an interesting section on how corpus material was used for the making of the dictionary.  More important though, it contains a comprehensive guide to effective communication from correspondence of many different kinds, link words and expressions, to using email and the internet. 

Monolingual dictionaries

Le Micro Robert is a dictionary of French language.  It is clear and user friendly, and gives a wealth of information on words including spelling remarks, grammar, analogies, synonyms and opposites, phonetics, register, prefixes and suffixes and verb tables.  It is a valuable aid for writing accurately.  This dictionary is more suitable for advanced learners with a knowledge of grammar.
Le Larousse, nouveau dictionnaire des synonymes provides a large choice of synonyms and gives differences of meaning, register of language, examples and grammatical structure. Best for advanced learners.
Le Thésaurus, Larousse is a tool that transforms ideas into words and words into ideas.  One can find words that relate to an idea and find ideas from words related to a notion.  It is organised according to themes and an index and makes a very valuable tool for advanced learners and francophones.
Le petit Larousse is an encyclopaedic dictionary.  It is composed of 2 main sections: a language section with more than 58,000 words plus locutions and expressions, and a general knowledge section covering history, geography, literature, the arts, science and technology. This large dictionary also includes almost 300 geographical and historical maps; it is illustrated and lists foreign locutions and proverbs.  It is a must for any francophile.