A new park recently opened up in Manhattan's West Side, called the High Line. Originally, the High Line was built in the 1930's to lift dangerous freight traffic off street level. It last ran in 1980. It was converted as some sort of revitalization effort, and the results (I think) are amazing. It is now a city run public space that provides both green relief and attracts hordes of tourists.
A similar idea was proposed for Toronto's Gardiner Expressway in downtown, which would put a roof on the current highway with a massive public park on top of it, nearly six stories up.