FreeRice is a website which allows you to feed the world whilst improving your vocabulary. You are given a word and four other words… one of which is a synonym. If you select the correct word, 10 grains of rice are donated to the UN. Not a lot but if you consider how many you can earn in two minutes, you’re feeding lots of poor people worldwide!
The words are by no means easy (how many laymen know what myalgia means?) and there’s 50 levels, but apparently it’s rare that anyone goes above level 48. In about 5 minutes, I’d donated 250 pieces of grain and I’m not even that super with the English language!
FreeRice also introduces you to lots of new words; improving your letters, blog posts or novels! 
They gain the money required to obtain the grains of rice through advertisements displayed each time you choose a word.
Over 2,400,000,000 grains of rice have been donated since 7th October 2007!
what is their revenue model, BTW?
Advertising displayed on the game pages
It’ll probably make them a small profit!