Free Rice

I love the thinking behind Free Rice. The idea is very simple, they use advertising profits generated by visitors to donate money to the UN World Food Program in an effort to help end hunger.  People are invited to come to the site and play a multiple choice word game.  For each correct answer, 20 grains of rice are donated.  The game strengthens your vocabulary and keeps your attention long enough to make the advertising money needed to pay for the rice donations.  It's super simple, super smart, and by the look of the stats, super effective.  (Found through notcot.org).