Via: autoblog green
"The debate over serial and parallel hybrid rages on, with Toyota and Honda placing their bets on the parallel model where both the gasoline engine and electric motor power the wheels directly. General Motors has placed a huge bet on the serial hybrid approach, where the second-fiddle powertrain serves only to extend the range of the predominately electric vehicle. There are numerous ideas as to what makes a good range extender. Hydrogen fuel cells and small gas or diesel engines are the most popular choices, but have you ever considered just using your legs?
Erockit has made a new electric bike which is capable of going fifty miles per hour and reportedly has strong acceleration. The range can be extended by pedaling along, though your strokes don't actually ever make it to the wheels in a conventional way. Instead, your legs turn a small generator which adds a bit of juice to the battery. The company has plans for a limited model release starting next year."