Squatting
Squatting is when someone starts living on empty land or in an empty building (usually a residential building) which the squatter does not own, rent or have permission to use. Author Robert Neuwirth suggested in 2004 that there were one billion squatters in the world.
Most squatting is residential. Squatting usually happens when a poor and homeless population sees old property or land.
Freetown Christiania is enough popular place for squatters.
By Country
United States
In the United States, squatting laws are different from state to state and city to city. For the most part, it is not tolerated for long, especially in cities.
Community organizations have helped the homeless to take over empty buildings. It is done not only to change the building to be a place to live but also a part of bigger campaign to let people know about inequality in housing. They also do it to help get change in housing and land issues.
References
Other websites
Radical agenda
Special issue of Mute Magazine on global slums
Crime
Anarchism