According to Motor Authority, Volvo has been testing a prototype KERS flywheel system hooked up to the rear wheels of an S60 T5 FWD. The flywheel is powered by the force of braking and can rev up to 60,000 RPM. On take-off, that power is channeled to the rear wheels with up to 80 hp of thrust. In testing, the 0-60 mph time was reduced by 1.5 seconds and fuel economy was boosted by 25%. Not bad for a 130 lb. unit.
See Extreme Tech’s article for more details.