Improve Your Vocabulary & Feed The World

This is one of my new favorite sites. Through a clever system supported by un-obstructive ads you can improve your vocabulary and for each word you are tested on ten grains of rice are donated to people suffering from hunger.

You start at a certain level of difficulty and as you answer well the difficulty climbs, when you fail you go back to another level. The site keeps track of the global stats for each word and determines the difficulty accordingly.