I used to play the FreeRice game a lot, and recently rediscovered how much fun it is.
Basically, you answer trivia about the meaning of words. You can do it in several languages, if you happen to be practicing for an language exam or just curious. Every time you get an answer right, they donate 10 grains of rice to the World Food Programme.
It's not a scam, but funded through the sponsor ads on the bottom of the page. You are never asked to pay for anything. It's fun for a lexophile like me, and extremely addictive. You're also doing good in the world.
Check it out! My highest level ever was 45. Anyone else do better?