Updated June 2021
Wordreference is the dictionary teachers use most and I believe it is still the best. It offers translation to and from French (plus several other languages). When you search a word or phrase it comes up with a good list of options. It also has a members' forum which you can consult for harder translations and the most up to date usage. Its creator, Michael Kellogg, says: "I started this site in 1999 in an effort to provide free online bilingual dictionaries and tools to the world for free on the Internet. The site has grown gradually ever since to become one of the most-used online dictionaries...". Recent upgrades, including the use of predictive text when searching, have improved the site further.
Linguee . "...a unique translation tool combining an editorial dictionary and a search engine with which you can search through hundreds of millions of bilingual texts for words and expressions." Recently greatly enhanced, it can work well on whole phrases but beware: it sometimes uses source texts which themselves have been translated, so it is not always reliable. Try typing in whole phrases or idiomatic expressions to see for yourself. To my mind, WF is still the best.
Larousse has five different monolingual French dictionaries including synonymes, antoyms, quotations, idioms, expressions, homonyms and verb tables. When you enter a French word you get a drop-down choice including definitions, synonyms and idiomatic expressions. There is also a basic bilingual dictionary accessible from the same page.
PONS.eu is a clear and effective bilingual dictionary.
Lexilogosis an excellent place to start as it is a portal to a range of dictionaries, both bilingual and monolingual (including Collins, Cambridge, Ultralingua and Wordreference).
Interglot is an effective bilingual dictionary which offers a large range of alternative translations.
Lingro is an easy to use multilingual dictionary which provides good coverage and claims to be the fastest on the web. More interesting is its facility to instantly translate words from a web page you choose. You type in the URL of the website, then you can just hold the cursor over a word and the translation is given.
Sensagent (Alexandria) is another dictionary which you can download and use in Word and Windows as a button on your toolbar.
Freelang.com produces detailed translations from English to French or French to English.
Ectaco is a bilingual dictionary which gives a good choice of translations. Messy page though.
The Dictionnaire Internationalis a very clear and quick monolingual dictionary. If you type in a verb it also gives you complete conjugations.
L'Internaute is a very good monolingual dictionary which also has alphabetical lists of words by theme, for example history, law and politics.
The CNRTL (Centre National des Ressources Textuelles et Lexicales) is a highly detailed dictionary with lots of examples of word use from literature and history. Very instructive if not easy on the eye. The search system only seems to handle entries of individual words which is a disadvantage. However, you can customise the dictionary and search via word classes. There are also sections for etymology, synonyms and antonyms. The latter two show bar charts to indicate the degree of sameness and differentness. The homepage says: "Créé en 2005 par le CNRS, le CNRTL fédère au sein d’un portail unique, un ensemble de ressources linguistiques informatisées et d’outils de traitement de la langue.
Le CNRTL intègre le recensement, la documentation (métadonnées), la normalisation, l’archivage, l’enrichissement et la diffusion des ressources.
La pérennité du service et des données est garantie par l’adossement à l’UMR ATILF (CNRS – Nancy Université), le soutien du CNRS ainsi que l’intégration dans le projet d’infrastructure européenne CLARIN.
The TLFI is a detailed monolingual dictionary.
Le Dictionnaire is a monolingual dictionary which is clear and attractive, though not particularly detailed. It prefers you to search with single words which can be frustrating if you have a phrase to translate.