Barcelona has a recently new program called Bicing, which is a bike sharing program. Bike stations are placed strategically throughout the city, and subway stops usually have signs pointing to them. Members have a card which they swipe, they're told which bike to grab, and off they go. It isn't necessary to return the bikes to where you got them; they can be dropped off at any bicing station. The first half hour is free, and after that there's a bit of a surcharge. When the program began they set the annual membership fee at some absurd low figure like 7 Euros, causing most of the city to sign up. That's now been increased to 24 Euros.
It's apparently become quite popular with seniors, who use them for leisurely afternoon bike rides.