This is an attempt to log the locations of ghost bikes, whether they are on permanent or temporary display. You can find a good reference site for locations to mark on Ghostbikes.org
From Wikipedia:
The first ghost bike memorial project was in St. Louis, Missouri, United States in October, 2003. After observing a motorist strike a bicyclist in a bike lane on Holly Hills Boulevard, Patrick Van Der Tuin placed a white-painted bicycle on the spot with a hand-painted sign "Cyclist Struck Here". Noticing the effect this had on motorists in the area, Patrick then enlisted the help of friends to place 15 more "ghost bikes" in prominent spots in the St. Louis area where cyclists had recently been hit by automobiles.
Similar projects began in Pittsburgh in 2004, New York City, Seattle in 2005 and Chicago in 2006. London Ghostcycle was active in 2005 and 2006, now defunct. There are or were projects in dozens of other cities worldwide.
In 2002 San Francisco artist Jo Slota started a similar, project which he called Ghostcycles. This was a distinct, purely artistic project which involved painting abandoned bikes white, and recording them on a website. Any connection between the two phenomena is uncertain.