Wordnik is a very interesting site. It's a place where one would go to learn any information on a given word.
For example, if you typed in the word, "web 2.0" it would give you results in the following ways: images, used in a sentence, related words, statistics, etc. Ideally, I can see this being used w/ L.A. classes or those wanting to help learn words.
Another unique feature to this site is the ability to register and then create your own content. So there is definitely a "web 2.0 vibe" going on here as well. Plus, there is a nice little feature which tells you how many times the word has been viewed and you can even click on an audio button to hear the word spoken (great for Spelling Bee practice).
For more info click here.