Holy applesauce! I went over to Freerice.com on Orbital Teapot's recommendation, and I have to say that it is so engrossing that I may not get my book done on time. Basically, they quiz you on vocabulary, and for every correct answer, the advertisers donate 10 grains of rice to the UN World Food Program.
Seems pretty straightforward. But, they've got some algorithm that measures how you're doing and then matches your level. This means that, at least for me, it was just the right ratio of challenging to easy that meant I couldn't look away until my eyeballs fell out of my skull. Plus, the little rice bowl is filling! How can you just leave when people are hungry? Okay, I'm going back. Book, schmook.