Wordnik: don't call it a dictionary

Wordnik is a bit like a dictionary, in the sense that it collects words and their definitions, but it aspires to be more than that. Users contribute example sentences, audio pronunciations, images from Flickr and more, so you can really get a good idea of how and when a word is used. There's also a thesaurus-plus "related words" feature, which shows not only synonyms and antonyms, but other words that are used in a similar context.

Wordnik isn't exactly Wiktionary, but it does accept user contributions of just about anything you can think of about a word. The combination of images, audio and contextual clues would also seem to make it a better dictionary than most for non-english speakers, and its library contains a weighty 1.7 million words. Since it's laid out better than most dictionary sites, and also incorporates definitions from some of the better ones, it wouldn't be a bad move to replace your current favorite online dictionary with Wordnik.