Word Geeks
I recently refound this site:
http://www.freerice.com/index.php
The idea is that you play and get 5 grains of rice per vocab word that you get right. You keep collecting and sponsors buy rice equalling what you've "donated".
I think it's an awesome concept. And, it's fun to play! I've been doing it for a couple of days again and I can already see a spike in my vocabulary.
So, by all means - check it out!