The Free Dictionary This is a dictionary I love. You can find synonyms, related words, antonyms and (this is what I love) references in classic literature.
Webster Definitions. Synonyms. Images. Usage frequency. Translations


Cambridge Definitions, American English, Idioms, French/English, Spanish/English
Oxford Dictionary

Social Dictionaries

Wiktionary Especially if you are looking for new words.
Lingro Collaborative dictionary. You can open an account and your searches are stored. You can organize them into different wordlists.
Leximo A “Social Dictionary” that invites users to submit words in every spoken language of the world.

Visual Dictionaries

Merriam Webster
Shahi A visual dictionary that combines Wiktionary content with Flickr images
Word Ahead Video vocabulary builder.