Club Penguin is an incredibly popular penguin-based MMO aimed at kids. Each player has a customizable penguin avatar and can wander around a large map. There are some mini-games which are fun, but the main emphasis of Club Penguin is the social aspect. Each player gets their own igloo which they can hang out or throw parties in as well as decorate with in game items, in-app purchases, and prizes from contests.
Since it was designed for kids, parental verification is needed in order to play.