Cambridge Learner's Dictionary. Designed for English language learerns.
Dictionary.com: After looking up a word, scroll to the bottom of the page: You may find the definition of the English word in your native language.
Longman ESL Dictionary: To learn how to best use the dictionary, click "how to use."
Merriam Webster Dictionary: Type a word in the Online Dictionary box, and hit "search." When the definition comes up, click on the speaker icon to hear the word pronounced.
Merriam Webster's Learners Dictionary: (For ESL students) Subscribe to a free "word-of-the-day" lesson, do pronunciation practice, and more.
Visuwords: A remarkable graphical dictionary. Type in a word and view the results in colors, shapes and lines.