This is one of those websites that I can't believe I didn't think of first: Wordnik is a website on which you can look up words. But it's not just a dictionary:
At Wordnik, you get:* real example sentences to show words in context
* meaningful information about your word's frequency and use patterns
* related words—not just synonyms and antonyms, but words that behave in similar ways
* the chance to contribute to our knowledge of English through recording pronunciations, pointing us towards new words, adding tags and related words, and leaving your notes.
It's part dictionary, part web-tymology, and part wiki: I especially like the feature that shows real-time Twitter usages of the word and the graph that shows how often the word has been used in the last two hundred years. I chose the word "oblong" out of the blue and here's the graph and frequency for "oblong":

I want to type in every word I know*.
(Via.)
* Dear commenters who hate me: Allow me to write this comment for you: "That oughta take about twenty minutes or so." Haw!
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When Pekin was plundered the Harems contained a number of balls a little larger than the old musket-bullet, made of thin silver with a loose pellet of brass inside somewhat like a grelot; these articles were placed by the women between the labia and an up-and-down movement on the bed gave a pleasant titillation when nothing better was to be procured. — Arabian nights. English
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