Wordnik calls itself "the most comprehensive dictionary in the known universe," and while that sounds like it might teach you a lot (it can) it doesn't quite give an indication of how much fun it is. The site aims to give you as much information as possible about any given word. Which means it doesn't limit itself to definitions. It gives you example sentences, related words, images, statistics related to usage and associated punctuation, reader-provided information, and even a list of tweets (from Twitter) that include your word. Type in "butt." Type in "existential." Type in your own name! The example sentences that appear might come from sources related to business, the arts, pop culture, even your favorite literary journals. The site is still in its formative stages, so not every word is there yet, but I'm looking forward to watching the site flesh (a fun word to Wordnik) itself out.