FreeRice

This is quite a simple game: you are given a word and have to pick the correct definition, if you get it right you score 10, otherwise you score 0, repeat ad infinitum. However for each point you score the FreeRice people donate a grain of rice to third world countries.
The game tracks how verbose you are too and assigns you a different difficulty of word depending on your past ability. Each time you get three answers in a row right you go up a level, each time you get an answer wrong you go down a level (nb. this only kicks in after you've answered a few questions). My average level was about 29/30, and my maximum level reached was 37. The difficulty of each word is determined by how many people got it right and wrong, and so may change over time too.
The rice is paid for by the advertising on the site, and is donated to third world countries via the United Nations World Food Program.