This application for iPhone or iPod Touch makes it easy to consult the online French dictionary TLFi from the “Centre National de Ressources Textuelles et Lexicales” (http://www.cnrtl.fr/).
It is possible to view the definition, the etymology, the synonymy or the antonymy of a word.
The TLFi (Trésor de la langue française informatisé) comes from a dictionary in sixteen paper volumes which contains 270 000 definitions and 430 000 examples.