http://www.poptropica.com/

Ages 6--15 Years
Free -- $3.95/Month

Poptropica® is a virtual world (online game) in which kids explore and play in complete safety.
Every month, millions of kids from around the world are entertained and informed by Poptropica's engaging quests, stories, and games.

Kids create a "Poptropican" character to travel the many Islands of Poptropica and use gaming literacy to enjoy a narrative that is often rooted in factual history.
Problem-solving skills are honed as kids discover and solve mysteries unique to each Island.

Poptropica is always free for kids to play.
However, the Poptropica Store offers membership extras that allow kids to personalize their character.
Paid memberships are as little as $3.33 per month.