Beef up your vocabulary and save the world at the same time! FreeRice donates 10 grains of rice to the World Food Program every time you get a word right. Not only will you be helping out the needy, which is its own reward, the wordgame is actually pretty addictive.
If you set up a profile on the website, it’ll keep track of all the words whose definitions you knew, and those you missed, so the test adapts to your vocabulary. The more words you get right, the harder the words it’ll throw at you, making it one of the better vocab tools available.