I've long used the online English-English/English-Chinese dictionary provided by Microsoft, often looking up words and putting the links in the chatbox during classes. It used to be at engkoo.com (and the Chinese name is still that) but a while ago it changed to Bing.
It is a free online dictionary that provides definitions, pronunciation and example sentences, which are great for seeing the word or phrase used in a variety of contexts. The example sentences also have Chinese translations and source links. You can choose to view definitions in English (English-English) or Chinese (English-Chinese).
One feature I especially like is comparison: you can look up and compare two words (or even more!) by putting "vs" in between. Especially useful for understanding the difference between similar words. Here's an example: cheap vs inexpensive.
It's a great all-in-one resource, although I still often use some more specialized sites when giving links for students. E.g. for example sentences, Jukuu often provides more results. And for pronunciation, I prefer Howjsay as it is all done by a native English speaker (whereas on Bing many are not).
Because of the name change, I hadn't got around to putting on my Links During Online Classes page, but it's there now, along with Jukuu, Howjsay and others.
G.A.L.E.S.L. / joe3
Some tips and links on learning English.