Learning in a Virtual World--Whyville

Whyville is an educational virtual world created for kids, which launched in 1999. It was one of the first solely for children and pre-teens and remains one of the few whose purpose is purely educational. Players earn "clams"—the currency of Whyville—by engaging in various games and role-playing scenarios. Players do everything from helping to publish the community's newspaper, The Whyville Times to running the power grid for Whyville to even taking out loans to buy big-ticket items such as cars. It is quite literally a self-organized community of kids that has been up and running for more than a decade. 

Whyville has received a number of awards from independent parent groups for its educational value and safety--and is reported to be one of the most popular virtual worlds (Wikipedia)