Here is one I came across recently: http://words.grammarly.com/
- Not as nice as the visual dictionaries on offer, but it is pitched at a level that EFL students could use.
- All of the words have definitions, and most of the more common words will have examples to show the sense of the word in context.
- Really easy to navigate and use.
- Also, dead easy to provide a link in any online feedback to writing, as the URL can simply be edited with the word in question, e.g. http://words.grammarly.com/words/job -- if your students submit their work online, you could easily hyperlink some of the main words that they are using incorrectly to have a look for the more appropriate synonyms.