This online translator offered by AltaVista has been around since the early days of the web.

Google Translate

Excellent dictionary with translations: www.answers.com

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This online French language tool offers 5 functions in one:
- French definition of a French word
- French synonym of a French word
- French conjugation of a French verbs

- French to English translation
- English to French translation
http://www.dictionnaire-mediadico.com/
alternate location:
http://dictionnaire.tv5.org/

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Probably one of the most complete dictionaries of the French language in a partnership with the notorious "Académie française". Sorry it's all in French and not very user-friendly. But this can be helpful for those rare words not found in other dictionaries. http://atilf.atilf.fr

Orthonet: http://www.sdv.fr/orthonet/
This site is not really a complete dictionary but it is fault tolerant and will try to propose the proper spelling of words.

Tools offered by the Québec government:
http://www.oqlf.gouv.qc.ca/ressources/gdt_bdl2.html