in college we did an informal Bike Sharea bunch of people in our dorms had beach cruisers that they were not using
so... we put them to use
they were cool with it
since we did not use locks
we often left the cafeteria or the party only to find that our someone else decided to take a turn on the bike
NYC has a more techie resolve to this
bike sharing in nyc on the homebrew level
www.crunchgear.com