Most of the hybrid vehicles in the world (and all of them for commercial sale, at this point) are electric hybrids. By that, I mean that propulsion comes from electricity and something else (typically an internal combustion engine). There, are, however, other hybrid technologies out there, some of which are starting to get more attention.
Some years ago, Ford was playing with an "air hybrid." The idea was that they had a large air tank somewhere in the car, and the engine had electrically actuated valves which could take in air from the intake or from the air tank, and output exhaust to the exhaust manifold or to the air tank. Basically, if you wanted to slow down, turn off the fuel and ignition and use the engine as a big, two-stroke air pump (air comes in thorugh the intake and is compressed into the air tank). When you are ready to pull off the light or climb a hill, use the engine as a two-stroke air motor (pressurized air from the air tank is fed to the intake on the engine, pushing the cylinders down, then simply ejected through the exhaust manifold; no fuel or ignition). Yet, when you run out of air pressure or you want to cruise, use the normal four-stroke system with the normal intake, exhaust, fuel and ignition. In this fashion, they were able to leverage a large number of existing components; the only real addition was the extra connectivity with the air tank and the electrically actuated valves which let you un in the engine in a two-stroke air pump or air motor mode, or four-stroke conventional mode.
I've read about it, but I'm having a hard time finding any current links on the subject. If you find any, feel free to bring them to my attention.
This recently got some more attention, when a Korean company suggested a car with batteries and an electric motor providing the main cruise power, and compressed air providing a hybrid mode for starting/stopping. Batteries don't handle large surges in input or output very well, so let the compressed air system handle that, and let the batteries do what they do best (slow, steady power).
Another type of hybrid technology is a hydraulic hybrid. This one requires adding some components to the drivetrain (a hydraulic pump/motor), some storage (called accumulators), and a variety of valves, lines and a controller.
What's an accumulator? Think of a long hollow cylinder with a piston in it. One end has hydraulic fittings and lines on it, and the other end has some kind of compressed gas fitting on it (nitrogen gas is popular). If you have no hydraulic pressure one one side, and about 1,000 psi gas pressure on the other side, that will push the piston all the way down by the hydraulic fittings. That's considered "pre-charged." Now, put some hydraulic fluid, under pressure, through their fittings. This will push the piston away from the hydraulic side, compressing the gas to even higher levels. The aircraft I've worked with usually had about a 1,500 psi pre-charge, and the hydraulic system usually ran about 3,000 psi. With full pressure on the system, that would push the piston about 1/2 way down toward the gas fitting, compressing the gas to the same 3,000 psi as the hydraulics. If you then closed the hydraulic valve, you could store energy in this fashion for days at a time. We used them extensively in the F-16. Consequently, it's relatively lightweight (you don't want to put a lot of heavy stuff in an aircraft), it's reliable (I never replaced a single one in four years as a jet mechanic, nor am I aware of anyone else who had to), and it can store a significant amount of energy (ever tried turning over a jet turbine? The F-16 uses stored hydraulic pressure to start a small jet engine, which then starts the main engine).
As with electric hybrids, there are both series and parallel varieties. The parallel variety is sometimes known as a "hydraulic launch assist." The basic idea is that, when you are braking, the valves are set so that hydraulic fluid is being pumped into the accumulators. This provides mechanical resistance, which slows you down, and stores the energy. When you're ready to get off the light or climb a hill, pressure from the accumulators can turn the hydraulic pump (which is now functioning as a hydraulic motor) and provide power for this. Normally, the systems envisoned cut out at around 30 miles/hour, but that's enough to damp out most stop-and-go traffic.
The series variety has a motor turning a hydraulic pump, some accumulators, and a hydraulic pump/motor turning the drive shaft. Such a vehicle typically doesn't need a transmission, which is a nice feature. I've found less coverage on this type of system.
Early in my Air Force days, I was talking to a friend about making a vehicle with four-wheel independent power, and no drive shafts or axle shafts. Put a hydraulic motor in each wheel hub and tie a hydraulic pump to the engine. I wasn't too worried (at that point) about regen braking, and I didn't understand accumulators well enough to think about adding regen braking to the system. My friend (Carl) liked the idea; he had a late '60's Mustang he was putting together, and it sounded like a cool idea. I talked to some more experienced mechanics, though, and they said that, while the idea was reasonably sound, the fly in the ointment would be the hydrodynamics. Getting the pressure to each of the wheels wouldn't be too difficult (hydrostatics), but I'd need a certain number of gallons-per-minute to maintain power at highway speeds, and that would be difficult. I'd need some really fat, heavy-duty tubing to carry it, very careful attention to bends in those lines (sharp bends tend to cause severe resistance to fluid flow), etc., all of which would be heavy and would generate a lot of heat (more gpm = more heat), which would seriously impact the performance and efficiency. They knew more about the subject than I did, and the obstacles sounded pretty overwhelming, so I dropped the idea (it took some SERIOUS explanation before Carl finally gave up on the idea, as well). If engineers are actually working on highway-capable hydraulic series hybrids, someone must have found ways around these issues.
In the meantime, I've seen and heard about a few other hydraulic-driven vehicles. Something they were fond of showing us in the Air Force was a film titled The Business of Paradigms. At one point, the narrator asked us what we would think of a small vehicle which got over 100 mpg. Most people can't even picture that. Then, we got to see the vehicle. It was a three-wheeler (a trike) which had a small motorcycle engine, turning a hydraulic pump, which fed to some hydraulic accumulators and a hydraulic motor which turned the rear wheels. The motor always ran at its most efficient setting, and the throttle would control fluid flow to the wheels. It also offered regen braking. While it only carried one person, and it couldn't do interstate speeds, it did exactly what the narrator described. I would characterize it as a hydraulic series hybrid.
Ever seen a skid-steer loader? They're better known as "Bobcats," even though Bobcat is only one of the companies which now manufactures them. The engine turns a hydraulic pump, the wheels are turned by hydraulic motors, and the pedals and levers simply control valves feeding fluid flow to the wheel motors and the forklift/loader mechanism. I wouldn't want to try driving one on the highway, though.
Also, while I was in the Air Force, I was acquainted with a machine known as a "jammer." Like a Bobcat, it had a small engine, which turned a hydraulic pump, which could operate a hydraulic lift or turn the wheels. Jammers were usually operated by "load toads," who used them to lift bombs (anywhere from 500 to 2000 pounds each) up and attach them to the bomb racks on the planes. Once again, a hydraulic series hybrid. I don't know if they had accumulators in them, though; I distinctly remember hearing the pitch of the engine change as they lifted objects or accelerated forward or in reverse.
I'm starting to wonder about the possibilities of some of these other technologies. Some of the newer hydraulic systems can handle over 5,000 psi, which makes them very power-dense. Also, pneumatic (air) systems are getting up to the 5,000 psi range, in no small part because people are looking for systems which can store significant amounts of hydrogen gas or natural gas (5,000 psi puts more energy in a cubic foot than 3,000 psi). The air-electric hybrid, in particular, is starting to sound VERY interesting. Too bad I never learned to read Korean (I spent a year in South Korea, but never learned the language).