Here’s a really nice project from New York, reclaiming a dis-used raised railway line and turning it into a park. It has been disused for a long time, and looked like this, a belt of green running through the city:
Although that’s kind of pretty in itself, it’s now been designed, made accessible, and acts as a shortcut for people walking. Visit Inhabitat to see what they did with the place.
London has a very similar disused railway line between Highgate and Alexandra Palace. It is a four and half mile long nature reserve that cuts right through the city.
The Earthbound Report 
Paris has one too, with part of the area below turned into artists’ ateliers.
http://www.paris-walking-tours.com/promenadeplantee.html
Luton has one of these as well, except it’s unofficial. Used by homeless people, drugies, youth and me. A very peaceful distraction form a busy town.