Wordnik
Wordnik is a dictionary aimed at including all kinds of words, regardless of how well they have permeated the language. It's kind of like a Wikipedia for words. Not only that, the definition pages are very comprehensive-- you'll find example sentences, tweets from Twitter that contain the word, pictures from Flickr, audio pronunciations, user comments. It's very new and very exciting. At the moment, the Wordnik dictionary has around 1.7 million words.