Academic Word List
This list has 570 words from a well-known New Zealand study of English language texts. This site has the list and exercises with the words broken into 10 lists.
Word Ahead Vocabulary Videos
These are like powerpoints with audio, and the vocabulary is quite advanced, but there is a nice audio definition at the bottom of the page, with printed text to follow. The vocab list is alphabatized, and there are now mobile apps (mostly SAT and GRE vocab) that you can download as well.
Create your own flashcards for free practice online
Spanish English cognates
We have lots of nationalities and languages in the classroom, but those from latin language groups can gain a wide vocabulary just by being aware of their cognates in English, and how their suffixes change to English ones. Most people aren't even aware of their own languages, and this gives them a quick vocabulary enrichment boost!