Do you let your students use an online translator, such as Google Translate or what was once the leader of the pack, Bablefish? I don't. EVER. And I feel very strongly about this, in case you couldn't tell.
The dictionary that I recommend to my students is Word Reference. But essentially I tell them if they see the word "TRANSLATE" anywhere on the web page, it's probably not a resource they ought to be using!
Aside from the ability to look up a word in French to find its English equivalent(s) and vice versa, there's a whole other aspect to WordReference that I just ADORE! I don't routinely show it to my core French students, but I introduce it to some that really strive to learn, as well as the type that will turn to a translator to express themselves given no other option. And I've definitely been showing it to my immersion students this year. At the bottom of any definition page, there are links that will show where in the forum the word you are searching has been discussed in a variety of settings.
The 'forum decorum' is that someone posing a question related to a particular usage has to try a translation themselves, and then others will pitch in to help them. I love that, because you know how I feel about collaboration in learning... I'm all for help, but love it when people are willing to try on their own, or to say "This is what I've got so far..." My goal is to leave them with some life-long skills for using their second language, even if it isn't perfect.
It's important to teach them how to use the forum links at the bottom of the page when they're looking up a word, for compound nouns as well as discussions. Too often, they look only for the first definition or two and stop there... students need modeling and opportunities to practice skimming the resource for the best match in the context they have.
I tell my French language learners that they need to search for a single word in a dictionary, and if it's part of a compound form or expression, to look below the standard definitions for that information. This is typically true in paper dictionaries as well.
What's your favourite online dictionary for second language learners?