Boy, I would hesitate to help myself to a bike that I saw laying around! I've been to Amsterdam many times and haven't noticed bikes for the taking (that you wouldn't risk being arrested for taking!)

I haven't seen that advertised but I had heard of it being offered in some cities. We just go to Mac Bike and rent them, including the insurance. Evidently, bike theft is a frequent occurrence. We once took a photo of a shiny new red bike that was locked down with 26, yes 26, different locks!

The "Witfiets" (white bikes) shared-use project has been tried a couple of times in Amsterdam, first in the 1960's and most recently from @ 1999-2002. In the 1970's they tried a Witkar (white car) scheme too. All have failed. Anyway, the white bikes scheme was largely designed for residents, not tourists, to use within half-hour increments - and most of the stations were in residential areas.

Bike theft is still a very big problem so always lock and anchor it. (locking both rims to the frame is recommended)
Keep in mind that locking a bike to a tree, bridge or lamp post is (supposedly) forbidden, giving authorities cause for removal. These sites will provide more info about this topic --

http://home.deds.nl/~quip/deel/witfiets.html

http://www.mit.edu/people/masam/e/bicycle/cases/depo.html

www.fietsendiefstal.nl
[click "english" at top of page]

http://www.cnn.com/2004/TECH/ptech/03/10/bikes.gps1.reut/

note: the 2nd site (mit) says there is another trial underway at the Waterlooplein.