The VocabGrabber, part of the Visual Thesaurus, is a nice resource for presenting and building vocabulary. Just copy in any text and the system will:
show the number of different words used
sort them by relevance
sort them alphabetically
class them according to subjects
show a thesaurus word web for any of the words you choose
present the sentence(s) the word appears in.
All of the words are hyperlinked, so your students can really explore meaning and collocations. I put in text from a blogpost of mine:
Using the Visual Thesaurus online is free. If you want to save and download wordlists, you must be a subscriber.