FreeRice.com is a great site which allows you to play an online vocabulary game to donate rice to the hungry. For each correct answer, the site will donate rice through United Nations. This is how it works: as you play the game, each page load brings up a banner advertisement which goes to pay for the rice. Read more about it in their FAQ.

Not only do you get to help donate food to the hungry, you can also improve your vocabulary! Each question gets harder or easier, depending on whether or not you are getting the questions right. So it plays to your own skill level and helps you learn new words. Try it out!