The UN World Food Program (WFP) has the site FreeRice.com where for each vocabulary question you answer correctly it donates 10 grains of rice to the WFP. The site is quite ingenious, it uses the money generated by the banner adds at the bottom of the page to buy the rice. It was created by John Breen and donated to the WFP.
The site is going beyond vocabulary into other subjects such as geography (identify country capitals and countries), identifying chemical symbols, art, language and math (pre-algebra).
They also have a nice video of the rice being distributed to refugees from Myanmar sheltering in Bangladesh.