Cambridge Learner's Dictionary. Designed for English language learerns.
Dictionary.com: After looking up a word, scroll to the bottom of the page for more useful information.
Longman ESL Dictionary: Type your word and click "search." The definitions are limited to common words.
Merriam Webster Dictionary: 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.
Thefreedictionary.com has definitions and some translations available
Google Images can work as a"picture dictionary" for many concrete words and ideas.
Visuwords: A remarkable graphical dictionary. Type in a word and view the results in colors, shapes and lines.