Arcology
An arcology is a term used to describe. The word "arcology" itself is a portmanteau of "architectural ecology," coined by Paolo Soleri, although the popular meaning of the term has broaded to cover several similar large-scale human habitats seen in fiction. However, they are not limited to the realm of science fiction, as several governments and private businesses are actively researching arcologies to put them into practical use as soon as 2010.
Advantages
Much like any planned city, an arcology can be built with an end result in mind, reducing costs. However, unlike most planned cities, arcologies are usual multi-story affairs, reducing travel time by adding vertical distance -- effectively dividing the distance needed to travel by the number of stories in the town. Because of this, an arcology using walkways instead of roads as the major source of travel.
Additionally, arcologies can be scalable. A single-building arcology could support the population of a small town (5,000), and additional arcologies could be built later in neighboring lots, seperated by walkways
Aquatic arcologies
Several designs for arcologies are designed to be built on sea, not land. This was inspired by the fact that several large ships (including tankers and nuclear carriers) are already equipped to spend months or years at sea, and (theoretically) could be built for increased or indefinate sustainability for populations.
One such design, Freedom Ship, has attracted relatively high amounts of media attention and criticism. Its design, similar to a barge's but at a much larger scale, would support a permanant population of about 25'000, including schools, a casino, a mall, an airstrip, and 100 acres of open 'land' (parks and recreation areas). The completeness of the plans has attracted investors, although not enough to begin construction.