Being an editor and a lover of words, Free Rice plays to my strengths, or at least my interests. I saw it first circulating on Facebook, among friends of similar ilk.
The concept is simple: You simply guess the right synonym for an obscure word, and if you're right, 10 grains of rice are donated "through the United Nations to help end world hunger."
If you're wrong, they give you an easier word, so you can get it right and make your contribution of rice. So, it accomodates those who aren't great with words but still helps them learn, and it makes the contributions possible at the same time.
According to their site, they are associated with a sister site, Poverty.com, and they goal is to do two things: end world hunger and end vocabulary poverty. As far as I can tell, the donations are funded by 3 advertisers that appear at the bottom of the game page, which refreshes with each play.
The goals are as simple and single-minded as the game itself. In a day when everything is complex and options are endless, simplicity is refreshing.