Tony Cassidy has compiled a great list of online Social Studies games.
I’d like to highlight three of them (though you should take a look at the whole list):
The Race Against Global Poverty is a virtual board game. You advance if you answer the questions correctly.
The Global Rich List isn’t so much a game as it is a way to compare yourself, and your income, with the rest of the world and see how you “rank.”
My Sust House has two games where you try to produce a more sustainable environment.
I’ve placed links to these games on the World History page of my website.