San Francisco rolls out new ‘smart’ parking meters that adjust their rates according to the supply and demand:

SFpark works by collecting and distributing real-time information about where parking is available so drivers can quickly find open spaces.

To help achieve the right level of parking availability, SFpark will periodically adjust meter pricing up and down to match demand. Demand-responsive pricing encourages drivers to park in underused areas and garages, reducing demand in overused areas. With SFpark, real-time data and demand-responsive pricing work together to readjust parking patterns in the City so that parking is easier to find.

You know, this whole free market / supply and demand thing could really catch on.