Others are more ambitious, requiring new construction or even an extensive rebuilding of city infrastructure consider what is needed to add a second set of pipes for a water-reuse system.

WSJ: How to Build a Greener City

The new building will reuse rain water to flush the toilets and a solar electric generating system will be installed on the roof.

BBC: Artist impression of the new building

Creating an Android app named "UltraReset" and using it in tandem with an NFC-enabled Android phone (a Nexus S, in this case), security researchers Corey Benninger and Max Sobell were able to reset and reuse -- free of charge -- transit access cards in both San Francisco's MUNI system and New Jersey's PATH system.

ENGADGET: Security researchers identify transit system exploit in San Fran and New Jersey, create app to prove it