While knowing how to string together words puts food on my table, FreeRice.com is trying to let you use such powers for the good of others with their vocabulary-testing site. FreeRice will service up a quick little vocab quiz with a word and four answers. If you answer correctly, the service donates 10 grains of rice to the United Nations World Food Program to give away to impoverished or hunger stricken people. If you're wrong, it'll let you know what the correct answer was and give you a chance at a new word while ramping down the difficulty. As you continue to progress the site will keep track of your "level" by changing the difficulty of the words for every three you answer correctly, which ends up getting pretty tough once you get above level 40 or so. The site caps off at 50, although you can continue to play as much as you'd like.
The site manages to work through sponsorships, whose tiny ads you'll see below the testing area. Since launching in early October, the site has donated more than 1.5 billion grains of rice, which it keeps a daily tally of here.