Free Rice http://www.freerice.com/
"Welcome to a secret weapon for encouraging vocabulary development and caring children families! Free Rice was originally developed as a method for helping vocabulary development, but now it also can be used for multiplication fact drill, chemistry, and second language practice. The site offers a word, then gives multiple choices for the meaning. A correct choice earns 10 grains of rice that show up in a wooden bowl on the side. Grains of rice are accumulated until 100 grains fill the bowl. The bowl is emptied and a running total is listed under the ever filling wooden bowl. As correct answers are given, the difficulty increases. An incorrect response adjusts the acceleration rate of difficulty and the incorrect word is added back into the rotation to be seen again. Groups can be set up to increase the rate of rice earned. The World Food Programme heads up the distribution of the rice earned by playing this game. To date, over 91 billion grains of rice have helped feed the hungry in our world."
Disclaimer: I did write the above blurb. I copied it from the OCTELA Newsletter Fall 2011.