It can sometimes be difficult to distinguish between an imaginary country, which does not even attempt to make any colourable claim to sovereignty, and a micronation, which does. To further confuse the issue, some micronations later convert to imaginary countries, such as Porto Claro.
There are also imaginary unions, such as the International Union of Mail-Artists (IUOMA). In mail art the union has turned into a real thing with their own site and their own discussion group.
See also: Aerican Empire, Fictional realm, IUOMA.