“Wordnik wants to be a place for all the words, and everything known about them.”

• An example sentence—even if we’ve only found one sentence for a word, we’ll show it to you. (And we’ll show you where the sentence came from, too!
• Related words: not just synonyms and antonyms, but words that are used in the same contexts. (For instance, cheeseburger, milkshake, and doughnut are not synonyms, but they show up in the same kinds of sentences.)
• Images tagged by our friends at Flickr: want to know what a “pout” looks like? We’ll show you.
• Statistics: how rare is “tintinnabulation”? Well, we think you’ll see it only about once a year. “Smile”? You might see that word many times, every day.
• An audio pronunciation—and you can record your own!
• Something YOU tell us! Use the “Contribute” links to tell us something—anything—about a word.

(from Wordnik About page)

Also check out this short screencast tutorial by Len Charnoff on using some of tagging and note taking features of Wordnik.