Catopia is a popular simulspace in which users become cats in old-Earth landscapes. There is no given goal, system of points, or even verbal or text-based ways to communicate with other players. Users often play hide and seek, tag, attempt new kinds of acrobatics, or just explore.
The setting is a series of small towns and villages in rural locations that could have existed almost anywhere on Earth. The presence of humans is implied by the distant sounds of vehicles, music playing from open windows, and the sounds of conversation from closed interiors, but none are actually present and the land belongs to the cats.
Variants using dogs, birds, rodents, monkeys and even spiders exist, but none have not gained the viral popularity of Catopia.