For a mild hybrid that just captures braking energy and uses it to restart the gas motor and pull away from a stoplight (but with very little range from regen recapture), a hydraulic pressure hybrid system is an interesting alternative to an electric system.
Some links:
NextEnergy - Hydraulic Hybrids
A New Wrinkle in Hybrids Does Away With Batteries - New York Times
http://www.epa.gov/otaq/technology/420f04024.pdf
EPA Displays the First Advanced Hydraulic Hybrid Vehicle
Ford Invents Hybrid that is 300% more efficient than Toyota Prius
http://www.designnews.com/index.asp?layout=article&articleid=CA6451735
Hybrid-hydraulic powertrain generates worldwide interest - 06/29/05
Hybra-Drive.com
Some links:
NextEnergy - Hydraulic Hybrids
A New Wrinkle in Hybrids Does Away With Batteries - New York Times
http://www.epa.gov/otaq/technology/420f04024.pdf
EPA Displays the First Advanced Hydraulic Hybrid Vehicle
Ford Invents Hybrid that is 300% more efficient than Toyota Prius
http://www.designnews.com/index.asp?layout=article&articleid=CA6451735
Hybrid-hydraulic powertrain generates worldwide interest - 06/29/05
Hybra-Drive.com