Wordnik is a great resource for teaching language as a topic within the English curriculum.
The Wordnik website focuses on showing how words are used by writers—not how they should be used, but how they are used, even if the use is what we might consider substandard. Wordnik is not intended to be used as a dictionary: It is for exploring the evolution of language in real time. Instead of just sentence or phrase, you will often get a whole paragraph to make the context clear.
The site is free, but you do have to register to use it.
A version of this article originally appeared in Writing Points for July, 2009, © 2009 Linda Aragoni