Ameba Pigg is a Japanese blog virtual community in which users can customize an avatar, socialize with people around the world using replicas of landmarks around the world like Shibuya 106 and the Statue of Liberty.
Users can also decorate their own rooms with different types of furniture, bought with virtual currencies. Users can and invite other players to their rooms and leave them messages. Some rooms are decorated by impossible constructions, which you can see below.
Images were found at http://amebapiggfaq.web.fc2.com/page/impossiblefigure.html.