Heading to the home of all knowledge, Wikipedia, we get a definition of:
A micronation, sometimes referred to as a model country or new country project, is an entity that claims to be an independent nation or state but is not officially recognized by world governments or major international organizations.There have been many reasons for the creation of micronations, however we are interested in their use as a tool to highlight alternative ways of governing. This could be through promoting a single issue, campaigning for a specific course of political action or an attempt to stop a perceived negative action.
Examples are:
Glacier Republic - ChileGreenpeace in an attempt to protect the glacier's of Southern Chile set a micronation to protest the Chilean governement toprotect the glaciers as a national asset.
Writland - Virtual LandWirtland describes itself as 'an experiment into legitimacy and self-sustainability of a country without its own soil'. It attempted to offer an alternative against nations conflicting over land and borders.
In response to the austerity cuts a mayor in Italy has developed his own micronation for a small mountain village. The aim is to develop an independent state, complete with its own currency, to battle the worst effects of the EU austerity cuts.