FreeRice donates 10 grains of rice to the United Nations World Food Program for every question you get correct on their website. You can choose from English vocabulary and grammar, famous paintings, chemical symbols, countries of the world and their capitals, languages and maths.
Banners advertising sponsors appear at the bottom of every question page. The money generated by those banners is what buys the rice. Since 7th October, 2007, FreeRice has donated 79,525,291,190 grains of rice. You can see the rice going to hungry people who need it on the website too. Would definitely be time well spent.