I can't remember having seen this on here before, but DH had to play this game for his geography class (in college) a couple of semesters ago. You can choose science, math, humanities, geography, and several other subjects. They donate 10 grains of rice for ever right answer. I've found that my boys feel like they're accomplishing something by playing. And they're motivated to try to get the answers right. http://www.freerice.com/category
On a similar 'websites doing good' theme there is What2Learn (find it in Google). This has thousands of learning games covering many subject areas so is really useful and it offers two kids of accounts. There are free accounts and Premium accounts. Half the money from the sale of premium accounts buys bricks to build schools in Africa.
