This site is one of the best I've ever seen for combining education with entertainment. It's a simple concept, elegantly executed. You're given a series of vocabulary words with multiple-choice definitions, and your level goes up or down depending on which words you answer correctly. There's even a charity aspect to it; for each word you get right, ten grains of rice are donated by the advertisers to the United Nations World Food Program.
The adaptive nature of the game means it's suitable and instructive for people with all levels of English ability; my four-year-old daughter plays at levels 1 (puppy, ship) and 2 (concentrate, employee) while my husband and I compete in the upper 40s (ratiocinative, adumbrate). I have made to 50, the highest level, only once.
I knew I'd be able to guess many definitions based on my knowledge of foreign languages, but I was surprised when I noticed that my Spanish was helping me far more than my Latin: arruga for rugose, nido for nidus, ala for alate. Then I realized it's simply because my Spanish is much better than my Latin; I just didn't know the Latin words for wrinkle, nest, or wing! In fact, they are identical or similar to their Spanish counterparts.
Enjoy! But be warned, the site is addictive!