It should be accurate and well made. Search history would be nice.
For a monolingual dictionary, the standard online reference is the Trésor de la Langue Française (TLF). You can search by pronunciation and browse the list of words. It's a reference dictionary, somewhat lacking in recent words.
A good list of resources for French from countries other than France is http://www.lexilogos.com/francophonie_dictionnaires.htm
Tennessee Bob's Famous French Links links to various resources on France and French.
CRISCO has a dictionary of synonyms.
For English-French bilingual resources, start with the Language Portal of Canada and EU portals such as the Translation and language help for small businesses and the Language resources and useful links for French.
For translations of technical terms, your first stop should be Wikipedia. Another good resource is IATE, the EU's multilingual term base.
Sure, there's quite a few. However my crystal ball is broken and I can't see clearly what are your needs. Check out the following ones:
- Wiktionary - quality varies, however sometimes very interesting information can be found
- WordReference - simple, yet effective (+ forums)
- CNRTL - good for tracking the history
This list is by no means exhaustive.
BTW. Why is this question (related to learning a language) in the WebApplications stack?
My favorite is: http://www.granddictionnaire.com
It has all of the very technical terms in any arcane domain you could think of.
Maintained by the Quebec government, which has to translate everything they do.
When you search for a term, it will show the definitions in all domains that apply (for instance: électroacoustique, robinetterie industrielle, génie mécanique, armée, etc).