Free Rice!

Free Rice is a site that tests your vocabulary, and for each word you get right, they donate 20 grains of rice through the United Nations to help end world hunger.

It's not easy. You start off at the default vocabulary level, and as you get words right, the level goes up. However, every time you get a word wrong, you drop a level. I maxed out at level 50 (the highest level in the game), but I didn't stay there long. I got 250 words right and donated 5000 grains of rice, and finished up at level 48. Give it a try.