The Japanese from Mazda have developed the ‘i-ELOOP’ – the world’s first capacitor based regenerative braking system for passenger vehicles which will start being used in production vehicles in 2012.
According to the automaker the i-ELOOP will be responsible for reducing fuel consumption by around 10 percent in real-world accelerating and braking conditions, and what makes it unique is the fact that it uses a capacitor instead of batteries in order to store larger volumes of electricity.
Unlike batteries, capacitors can be charged and discharged much faster and are more resistant to prolonged usage and deterioration. 
Mazda’s i-ELOOP contains a 12-25V variable voltage alternator, a low-resistance electric double layer capacitor and a DC/DC converter. When the driver lifts off the acceleration pedal and the vehicle begins to decelerate, the i-ELOOP starts to convert that kinetic energy into electricity which can be distributed to the vehicle’s electrical components. This way the vehicle does not have to brun extra fuel to generate electricity.
Mazda says the i-ELOOP which is an adaptation from ‘Intelligent Energy Loop’ can work in conjunction with their idling stop technology, the ‘i-stop’.

Source: Mazda