My ride in Copenhagen. The city provides bike stands in central Copehagen which each have ~10 bikes or so. You place a 20 DKK (~3$) coin in the bike's lock and can take the bike. You get your money back when you return the bike to one of the bike stands.

Perfect system because you can use the bike, and if you leave it it's not a big deal as it's a 20dkk recovery fee for whomever finds it/returns it.

Here's a catcher though - the police requires that you carry your own bike light. (I nearly got a ticket for riding one of these bikes without a light at night..)