Ponte Vecchio in Florence, Italy, Wikipedia Commons
Jeremy at Shareable writes about the glories of bridges: "By definition, a bridge connects one place to another, and thus seems to promise transition and adventure. Time seems to stand still as we make the crossing, wondering what new world might greet us on the other side." They also include some interesting inhabited bridges, with a completely different kind of transition.
At one extreme is Ponte Vecchio in Florence, originally occupied by "medieval butchers, most of whom lived above their shops. Today, it's a tourist trap where one can purchase jewels, art, and souvenirs."
At the other is Stephen Holl's proposal for an inhabited bridge in Copenhagen which "combines sustainable technologies like photovoltaic-sheathed skyscrapers and a wind turbine-studded skybridge with a commitment to mixed usage--the towers and bridge contain a public auditorium, offices, shops, skyways, and a terrace." (more coverage of this in Inhabitat)
We also learn of an inhabited bridge, Krämerbrücke, in Erfurt, Germany, that was built in stone in 1325 after burning down seven times since 1156.
TreeHugger has a fondness for inhabited bridges and has shown a few proposals; Laurie Chetwood won a competition to build a new London bridge that has everything a TreeHugger could love, including a vertical farm, a farmers market, photovoltaics, a wind turbine, cafés and shops. Rainwater is harvested to supply the hydroponic landscape and toilets. More at Proposal For London Bridge Has Solar Power, Vertical Farm
"Imagine housing, recreational and cultural facilities connected to a continuous, lushly planted, green strip, floating above the water - an aerial garden, as the cities newest park through which you could walk and wander and enjoy the most spectacular views of the bay." More at Bridges Are For People: Inhabiting the Bay Bridge
Antoine Grumbach designs a new London bridge with a pair of 35 storey towers holding up the bridge, which is lined with cafes, bars and shops, along with trees, a greenhouse and a "topiary café". More at Creative Financing: Putting Shops and Cafes Back on a London Bridge
Constant Nieuwenhuis developed a vision of New Babylon where everyone lived in bridges above the ground, so that it was left open for agriculture, nature reserves and forests.
"The question, how the people would live in a society without hunger, without exploitation, and also without labour. In a society thus in which every person without exception would be able to fully develop their creativity. This important and intriguing question calls forward the image of a material environment that substantially differs from everything that we can, from everything that ever was established in the area of architecture and town-planning."
See yet more inhabited bridges at OOBJECT