Are you learning English and are looking for an online dictionary? Or maybe you are a proofreader or copy editor searching for a good advanced dictionary? Here are a couple of useful links. They are not listed in alphabetical order, but in order of my personal preference. I tend to avoid dictionaries that have too many adds.
Word reference – Very useful dictionary. It searches the word in several other dictionaries, as Oxford and Collins. In addition the website provides over 12 different dictionaries, as English-French, -Spanish, -Portuguese, -German, -Italian, -Corean and so on.