Designer Anke Salomon's public monowheel concept.

Via: the design blog

"Keeping in mind the specific problems faced by tourists visiting Germany’s capital city Berlin, the City Bike by designer Anke Salomon is a bold new initiative that promises to make the most famous areas of the town more accessible for tourists. The Public Bike system will allow tourists to tour the parks and pedestrian precincts through this innovative rental bike that runs on two wheels that turn independently of each other.

With the seat holding the wheels in position while gliding within the wheels on ball bearings, the bike is propelled forwards by pedals that are used in a straight movement. Apart from handles that aid in steering and braking, the bike also features a compartment for hand luggage.
The drive system in the bike is based on a magnetic field with a permanent magnet fixed inside the seat and a number of electromagnets housed inside the wheels.

Since pedaling generates an electromagnetic current drives the bike forward, a very negligible effort on th part of the rider is required to make the bike move forward which, of course makes this bike a strong contender for a public bike rental service."