Free Rice
Free Rice is an online game that helps build knowledge about vocabulary and other subjects. According to their Web site:
"FreeRice is a non-profit website run by the United Nations World Food Program. Our partner is the Berkman Center for Internet & Society at Harvard University.
FreeRice has two goals:
  1. Provide education to everyone for free.
  2. Help end world hunger by providing rice to hungry people for free.
This is made possible by the generosity of the sponsors who advertise on this site.
Whether you are CEO of a large corporation or a street child in a poor country, improving your education can improve your life. It is a great investment in yourself.
Perhaps even greater is the investment your donated rice makes in hungry human beings, enabling them to function and be productive. Somewhere in the world, a person is eating rice that you helped provide."

In addition to building vocabulary, Free Rice provides opportunities to build knowledge in Art, Chemical Symbols (Basic), Chemistry Symbols (Full list), Multiplication, Basic Math, English Grammar, and Language Learning in French, German, Spanish and Italian.