FreeRice vocabulary game

I found a great vocabulary testing game called FreeRice, via a post on Gamers With Jobs. It gives you a word, and you have to pick it's synonym from a multiple-choice selection. Each correct answer donates twenty grains of rice to the United Nations World Food Program, who in turn give to the people who really need it.

It's a really nice idea, well implemented, and has a simple levelling mechanic, so words get harder as you keep playing. The FAQ has more on where the rice comes from, and how the levelling system works. It says there are fifty levels, with it being rare to go about 48. I'm hovering around 46, so I need some more practice :)