Call-A-Bike allows you to rent a bike for 4 cents per minute (US$2.90/hour). When you spot a bike with a flashing green light (I saw at least a dozen all over town on my three-hour walk around), you use your cell phone to activate it, then ride it wherever you want and leave it there for the next person.

For more information, see the Call-A-Bike web site. It's an interesting idea; I wonder whether it'll work.