I am completely addicted to freerice.com. I play around on it for days on end, then forget it exists for a few months, and then rediscover it and remember why I was hooked in the first place.
Not only are you spending time testing your own literary, science and trivia knowledge in clever ways, but you're also donating rice and money to those in need. So you end up smart and a philanthropist. Everybody wins!
What's your highest score? I can make it to level 43 pretty quick. but I can't seem to break through that barrier to the next level of word definition. The great thing about this site is, even if you get an answer wrong, it asks you again later so you can feel like you learned something. Sweet!