This (brief) New Scientist article covers the use of compressed air as an energy store for hybrid ICE vehicles. The advantage over a battery is the rate at which energy can be stored when decelerating.
http://www.newscientist.com/blogs/onepercent/2011/02/green-machine-hybrid-cars-as-l.html
http://www.newscientist.com/blogs/onepercent/2011/02/green-machine-hybrid-cars-as-l.html