Just got word of this site from a homeschool mailing list I read. It's called FreeRice, and it's a multiple-choice vocabulary game. For each word you define correctly, 10 grains of rice are donated to help feed hungry people worldwide. And the difficulty level adjusts to your performance, so you'll probably even learn something as you get frustrated. So hey, go forth and click...
(Helena, I am particularly eager to hear how you and/or Doug do on this one.)
(Edited to add: It is amazing how a working knowledge of Dungeons & Dragons and fantasy literature is serving me at this game. Tarn! Kirtle! Kobold!)
(Edited again to add: Thanks, Mom, for the childhood vocabulary lessons. I'm so happy that knowing what an ecdysiast is has come in handy.)