Here is one of the newer entries into this arena.
Instantly add electric propulsion to your bicycle with this self-contained, powered wheel
Design BenefitsSimple Installation
Install or remove from most bicycles in seconds, as a matter of routine
Simple power on demand throttle control
Maintain high speed up nearly any hill with one Hp direct drive motor
Fun to Ride
Fast and responsive, with low center of mass; hybrid using human and electric propulsion
Drive components supported by shock-mounted wheel, for minimal stress on bicycle frame
Silent, industry standard hub motor
US and foreign patents pending
A Truly Green Ride
Excellent energetic performance; 25 times more efficient than an automobile.
Gee, I'm sold.