Free Rice-- a site dedicated to building vocabulary and saving world hunger. This would be a great tool to offer extra credit...the more vocab you do, the more rice the website will donate through the United Nations to help world hunger.

Practice Vocabulary tests - great for classroom entry time
http://www.vocabtest.com/vw/index.htm

Try www.flocabulary.com. It is a site that uses rap & hip-hop to increase vocabulary. You have to purchase the whole set, but there are some excellent demos that are fun and helpful.