Well, I'm far from being Max (flieger) but technically the suspension is oleopneumatic. I have a fundamental understanding of the workings of such a system, but in the interest of time I will copy and paste the wikipedia version which is complete enough:
"In a hydropneumatic suspension system, each wheel is connected, not to a spring, but to a hydraulic suspension unit consisting of a hydraulic accumulator sphere of about 12 cm in diameter containing pressurised nitrogen, a cylinder containing hydraulic fluid screwed to the suspension sphere, a piston inside the cylinder connected by levers to the suspension itself, and a damper valve between the piston and the sphere. A membrane in the sphere prevented the nitrogen from escaping. The motion of the wheels translated to a motion of the piston, which acted on the oil in the nitrogen cushion and provided the spring effect. The damper valve took place of the shock absorber in conventional suspensions. The hydraulic cylinder was fed with hydraulic fluid from the main pressure reservoir via a height corrector, a valve controlled by the mid-position of the anti-roll bar connected to the axle. If the suspension was too low, the height corrector introduced high-pressure fluid; if it was too high, it released fluid back to the fluid reservoir. In this manner, a constant ride height was maintained. A control in the cabin allowed the driver to select one of five heights: normal riding height, two slightly higher riding heights for poor terrain, and two extreme positions for changing wheels. [The correct term oleopneumatic (oil-air) has never gained widespread use. Hydropneumatic (water-air) continues to be preferred overwhelmingly.]"
Citroën DS - Wikipedia, the free encyclopedia
This is an example of active suspension, or at least semi active if you consider what can be further achieved with electronic control of individual units. The 911 E had a 'hydropneumatic' front suspension that was self contained in the strut dampers and was not externally adjusted. For the sake of this thread I will say that is the passive version for lack of a better word.
Then there's the 959 but that's another story.
Max?