Forgotten to include this very noble and worthy site.
Started off as a simble vocabulary quiz site where every question you got right donated a number of grains of rice to the World Food Programme.
Now it has developed to include a range of subjects including History, Maths and Languages (amongst others).
You can play for free or create (tutor/year) groups and compete against each other on a leaderboard.
http://freerice.com/about/faq has all the answers you need
See on freerice.com