Max Vs. The Internet

FreeRice is a simple word game that is surprisingly compelling. Match words with their meaning, and for every word you match correctly, 10 grains of rice are donated to the United Nations World Food Program.

It's cleanly designed to the point of being stark, but with an immediate reward that closes its compulsion loop. Additionally, there's a nice sense of personal investment and challenge because the game grades and tracks your vocabulary level as you play.

Via Omniglot