Freerice.com is an intriguing site that claims to do two things: help you build your vocabulary and feed the world's hungry. I'm not certain of its effectiveness on either count, but it's an ingenious attention-grabber.
The premise is that you're given a series of words to define. Each word has four purported synonyms. If you pick the right synonym, you earn 20 grains of rice, to be donated to the United Nations World Food Program. The ads on the site pay for the rice.
When you miss a word, you're given an easier one. When you get one right, you're given a slightly harder word. The site keeps track of your score, too. I got up to level 50 a couple of times, but fell back to level 46 or 47 pretty quickly. At the upper levels, you're getting words like apologue, petrous, secern, axenic, furcula, altazimuth, and retrorse. Time for some educated guessing.