The Portland bike community has been installing ghost bikes since the summer of 2005. The local Ghost Bike Project was very active in 2005 and 2006, putting out memorials for many cyclists over the previous years. Since then, the process has become more organic and new people have been helping erect and maintain memorials.
They documented their local project, as well as ghost bikes in other cities, on their site ghostbike.org. In July of 2008, ghostbike.org and ghostbikes.org joined into one database of ghost bikes around the world.
Information on ghost bikes and memorial rides in Portland, as well as other bike news and events, can be found at BikePortland.
Extensive visual documentation may be viewed on Linda Kay Lund's Flickr here where she has dedicated a photo set just to Portland ghost bike memorials.