I just tripped upon this awesome site, FreeRice.
Here's how it works:
You take a never-ending multiple choice vocabulary quiz. For every question that you get right, you donate 10 grains of rice through the UN to help end world hunger. It's as simple as that. There's no cost. There's no limit to how long you can play or how many times you can visit the site.
It's exactly what it says it is: free rice.
Check it out!