http://www.freerice.com/
Pretty cool idea. Basically, there are quizes with a number of different subjects (language, math, geography ... elementary school type stuff). For every question you get right, 10 grains of rice are to be donated to the hungry via the United Nations World Food Programme. It doesn't sound like much, but it adds up pretty quick. So far, some 82 billion grains of rice have been donated (enough for millions of meals). You don't need to sign up, but if you do you can track your progress and join groups.
I started playing and I got 111 English vocab questions right in a row before I guessed wrong on some word I'd never heard before. Maybe I'll try chemistry next ...