Recommended online dictionaries for both foreign and native speakers!

Merriam-Webster Online
Merriam-Webster provides a free online dictionary, thesaurus, audio pronunciations,
Word of the Day, word games, and other English language resources.

Dictionary.com
Free online dictionary search, translator, word of the day, crossword puzzles and word games, and vocabulary learning resources for many languages.

Dictionary of Slang
A monster online dictionary of the rich colourful language we call slang... all from a British perspective, with new slang added every month.

Cambridge Dictionaries Online - Cambridge University Press
Free online dictionary from Cambridge University Press.

Online Dictionaries and Translators
Online dictionaries that help translation from one language to another language. Many languages covered.