airbags
A pneumatic spring often used in custom automotive suspension systems.  Airbags should not be confused with an airbag, the crash safety device.  Properly installed airbags offer better ride quality over conventional steel suspension springs and also allow for the adjustment of the vehicle's height, typically from a height where the vehicle chassis is literally laying on the pavement to somewhere near the original ride height of the vehicle.

Airbags may be installed in search of either low vehicle height, an improvement in ride quality, or a combination of both.  They were originally introduced for use on large semi trucks and city buses, but have gained a large following in the sport truck and minitruck scenes over the past 15 years.
The airbags on my truck make it possible lay frame on 22's, yet also lift higher than stock.