The purpose of this thread is not to start a debate about which AWD system is better. It is simple a thread to explain how the systems work and simple differences between them. Nobody is endorsing one system over the other, so please keep bias out of this thread.
This is also a Work in Progress (WIP). Please feel free to contibute so that it may become a source of information for all.
Types of AWD Systems
Subaru (USDM 5spd) & Mitsubishi (USDM and NOT EVOLUTION)
Subaru (USA 5 speed) and Mitsubishi (USDM) systems are made up of 3 limited slip differentials (LSD's). The first one, starting from the front to rear, is a limited slip differential. The middle (Center) and rear differential are both viscous limited slip differential.
The limited slip differential is a SPEED SENSING differential, which means that it has to detect one side of the diff spinning faster that the other to begin creating a torque bias. In this case you have a delay between wheel slip and actual torque shift from one spot to another. If we look at a current version of a Subaru or Mitsubishi, you have ABS which is a speed sensing system and the viscous coupling diff. which is also a speed sensing system. In most cases, your tires will slip long before you are able to transfer torque, which actually can save wear and tear on the differential and drivetrain.
Subaru (USDM Auto) and Audi TT and Jetta 4Motion
The drivetrain is made up of 2 limited slip differentials and a haldex or haldex type center diff. The Haldex center diff is more or less a multiplate clutch system that splits the torque between the front and rear driveshafts. The Haldex system is again a SPEED SENSING system. As one set of clutch plates spins faster than the others, torque is shifted. In normal operation, the Haldex diff will keep power split closer to 60(rear):40(front). As a slip occurs torque shifts more towards another end and can reach levels of 80% at the rear diff. Once again ABS is also employed to add another level of SPEED SENSING control over the wheels.
Audi Quattro 5 speed, 6 speed and Automatic
This type system incorporates a Torsen center differential and 2 open differential (front and rear differentials). The Torsen center diff is quite different from the other diffs mentioned above. The reason is that is uses TORQUE SENSING to determine the direction to move power in the drivetrain. A torque sensing diff can see torque changes almost instantly, even if torque levels shift 1% towards the other end of the car! This means that the very moment a tire looses it's grip, torque is starting to move away from that end of the car. If this wasn't cool enough, the Audi cars also have ABS that will pulse the brakes at each wheel when needed to take torque away from one slipping tire if needed. When this means to you is that you have both a TORQUE SENSING and a SPEED SENSING system on board your car.
Lancer EVOLUTION (UKDM/JDM) and Subaru WRX STi (USDM/UKDM/JDM)
These vehicles carry an electronic control unit (ECU) that controls the haldex system. The ECU's can determine how to push torque based on wheel sensors, as well as sensor readings of road speed and engine RPM. These systems are as fast as the Torsen system but use their independent ECU to manage torque split. On the STi this system is referred to as DCCD (Driver Controlled Center Differential) and on the Evolution it is referred to as ACD (Active Center Differential). The STi version includes a manual adjustment wheel to set the diff to any torque bias you want or there is an auto setting to let the computer decide for you.
A limited slip differential (LSD) is a modified or derived type of differential gear arrangement that allows for some difference in rotational velocity of the output shafts, but does not allow the difference in speed to increase beyond a preset amount. In an automobile, such limited slip differentials are sometimes used in place of a standard differential, where they convey certain dynamic advantages, at the expense of greater complexity.
The main advantage of a limited slip differential is found by considering the case of a standard (or "open") differential where one wheel has no contact with the ground at all. In such a case, the contacting wheel will remain stationary, and the non-contacting wheel will rotate at twice its intended velocity – the torque transmitted will be equal at both wheels, but will not exceed the threshold of torque needed to move the vehicle, thus the vehicle will remain stationary. In everyday use on typical roads, such a situation is very unlikely, and so a normal differential suffices. For more demanding use however, such as driving off-road, or for high performance vehicles, such a state of affairs is undesirable, and the LSD can be employed to deal with it. By limiting the velocity difference between a pair of driven wheels, useful torque can be transmitted as long as there is some friction available on at least one of the wheels. 
The viscous type is relies on the properties of a dilatant fluid - that is, one which thickens when subject to shear. Silicone-based oils are often used. Here, a cylindrical chamber of fluid filled with a stack of perforated discs rotates with the normal motion of the output shafts. The inside surface of the chamber is coupled to one of the driveshafts, and the outside coupled to the diff carrier. Half of the discs are connected to the inner, the other half to the outer, they alternate inner/outer in the stack. Differential motion forces the interlocked (though untouching) discs to move through the fluid against each other. The greater the relative speed of the discs, the more resistance the fluid will put up to oppose this motion. In contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, so for the average driver is easier to cope with.
Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. They do not stand up well to abuse, particularly any sustained load which overheats the silicone results in sudden permanent loss of the LSD effect. This is up for debate, but more or less reality. 
The Haldex center differential is mounted on the rear axle differential and is driven by the prop shaft. Engine torque is transmitted through the gearbox to the prop shaft. The prop shaft is connected to the input shaft of the Haldex differential. In the Haldex differential, the input shaft is separated from the output shaft to the rear axle differential. Torque can only be transmitted to the rear axle differential when the Haldex differential clutch plates are engaged.
In the absence of wheel slippage, the clutch plates are not engaged, and only front-wheel drive operates until power is needed for the rear axle.
1. Permanent all-wheel drive with electronically controlled multi-plate clutch
2. Front drive characteristic (distributes power to the rear axle only when needed)
3.Quick response time, No strain on clutch when parking and maneuvering the vehicle
4.Compatible with different tires (e.g. emergency wheel), No restrictions on towing with the axle raised off the ground
5.Fully combinable with traction control systems such as ABS, EDL, TCS, EBD and ESP. 
The torsen differential works by supporting a torque imbalance, the maximum ratio of torque imbalance is defined by the TBR (Torque Bias Ratio). When a torsen has a 3:1 TBR, that means that one side can handle up to 75% while the other side would have to only handle 25% of applied torque. During acceleration under asymmetric traction conditions, so long as the higher traction side can handle the higher percentage of applied torque, no relative wheelspin will occur. When the traction difference exceeds the TBR, the slower wheel receives the tractive torque of the faster wheel multiplied by the TBR, any extra torque remaining from applied torque contributes to the angular acceleration of the faster wheel.
When attempting to turn with a torque sensitive differential, the outer wheel will need to move forward relative to the differential, and the inner wheel will move slower than the differential. Friction in the differential will oppose motion, and that will work to slow the faster side and 'speed up' the slower/inner side. This leads to asymmetric torque distributions in drive wheels, matching the TBR. Cornering in this manner will reduce the torque applied to the outer tire, leading to possibly greater cornering power, unless the inner wheel is overpowered (which is easier to do than with an open differential). When the inner tire (which has less traction due to weight transfer from lateral acceleration) is overpowered it angularly accelerates up to the outer wheel speed (small percent wheel spin) and the differential locks, and if the traction difference does not exceed the TBR, the outer wheel will then have a higher torque applied to it. If the traction difference exceeds the TBR, the outer tire gets the tractive torque of the inner wheel multiplied by the TBR, and the remaining applied torque to the differential contributes to wheel spin up. 
Type of 4wd/AWD systems and sample cars
4WD and AWD systems by design type
NOTE! Subaru's "Symmetrical AWD" system is actually 4 different systems based on drivetrain choice, see below.
Center differential with mechanical lock, or other torque transfer features
Ford - Escort and Sierra Cosworth, Sierra and Granada 4x4 models
H1 & HMMWV NVG 242HD AMG open center differential, locked center differential, Neutral, low range locked. Also Torsen1 differential at the front and rear axle, The H1 moved to Torsen2 when ABS was added. The H1 Alpha had optional locking differentials in place of torsens
Hummer H2, H3 40/60 planetary with lock
Land Rover/Land Rover Discovery/LR3
Subaru Basic manual transmissions have a 50/50 center differential with viscous clutch, performance models have a planetary differential with computer regulated lockup. Automatic transmission models have ~45/55 planetary with computer controlled lock up
Mercedes GL class
Jeep Grand Cherokee, Commander (Quadra-Drive 2 version only for both vehicles)
Jeep Liberty (Select-Trac) NVG 242 transfer case-rear drive, open center differential, locked center differential, Neutral, low range & locked
Porsche Cayenne (Porsche Traction Management) 38/62 planetary with lockup clutch pack
Volkswagen Touareg -double pinion 50/50 with lockup clutch pack
Suzuki Grand Vitara
Toyota Land Cruiser
Toyota Sequoia (Multi-mode)
Lexus RX350 -viscous coupling on center differential
Lexus LX470 -open with lock
Mercedes-Benz Unimog (locking center and rear with up to 10 low range gears).
Mitsubishi Pajero (also known as Montero or Shogun)
Note of the above vehicles all have a low range transfer case except some Subaru models.
Torsen Center diff
Audi A4, A6, A8, S4, S6, S8, R8, Q7 (center) (quattro)
Chevrolet Trailblazer SS (center)(limited slip rear) Torsen3
Lexus GX470 (center)(limited slip rear) Torsen3 with lock
Toyota Hi-lux Surf or 4runner (center) (also locking rear) Torsen3 with lock
Toyota FJ cruiser (center) (only manual models) (also locking rear) Torsen3 with lock
Bentley Continental GT, Flying Spur Torsen1
Center diff no locking
Mercedes 4MATIC cars, R class, and ML class (note some MLs had low range)
Cadillac Escalade, STS AWD, SRX AWD (The first two generations had a viscous clutch on the center differential)
GMC Yukon Denali, XL Denali, Sierra Denali
Chrysler 300C AWD
Dodge Magnum, Charger AWD
The above systems function by selectivly using the tracton control system (via ABS) to brake a slipping wheel.
Multiple Clutch systems
Acura RL, RDX (SH-AWD) Right and left axleshaft
Nissan Skyline GT-R (ATTESA E-TS and ATTESSA E-TS-PRO) front axle coupling, rear differential locking
Nissan Skyline GTS4 (ATTESA E-TS)
Nissan A31 Cefiro SE4 (ATTESA E-TS)
Porsche 959 PSK front axle coupling, rear differential locking.
Acura MDX SH-AWD & VTM4
Mitsubishi GTO MR/3000GT VR-4
Mitsubishi Lancer Evolution Series
Clutch pack coupling
Subaru Low powered automatic transmission models: mechanical front drive, clutch coupled rear axle.
Volkswagen Haldex based cars
Volvo S40, S60, S80, V50, V70, XC70, XC90 (all Haldex based)
Mazda Tribute, CX-7, CX-9 (tribute Control Trac II, based)
Mazdaspeed6 (a power takeoff unit linked to clutchpack with torque sensitive rear differential.)
Ford Escape, Freestyle, Edge, Fusion, Five hundred (Freestyle, FiveHundred Haldex based)(Escape Control Trac II, based)
Mercury Milan, Montego, Mariner (Montego Haldex based)
Nissan Murano automatic with manual lockup swithch
Lincoln MKS, MKZ
Land Rover LR2 (also Haldex)
Audi A3, TT (also Haldex)
Jeep Compass (Freedom Drive)
Toyota Rav4 (latest model, not older generations)
Honda CR-V, Element
Chrysler Pacifica (BorgWarner ITM3e) (on 2007 model)
Hyundai Santa Fe (BorgWarner ITM3e) (2005 and up), Tucson (also a BorgWarner system)
Kia Sportage (also a BorgWarner system),
Jeep Grand Cherokee & SRT8 NVG 249, 247
Dodge Nitro (Quadra-Trac 1)
Suzuki SX4, XL7, Aerio
Mitsubishi Outlander (current generation)
BMW 3series, 5series, X3, X5 (the initial X5's had a 38/62 planetary center differential)
Chevrolet Equinox (GMPCA)
Pontiac Torrent (GMPCA)
Infiniti G35x, M35x
Subaru low powered automatic transmission models
Porsche 911 Awd variants (a version of BorgWarner ITM3e) -excluding the 964 series Porsche 911 carrera4 31/69 planetary center differential
Lamborgini AWD variants VT series (viscous traction)
Note the above all function like 2wd when clutch pack not engaged, and like 4wd highrange in a part time 4wd system when the clutch is engaged (usualy by computer although some allow manual control). Some in this category have varying degrees of control in the torque distribution between front and rear via alowing some of the clutches in a clutch pack to engave and slip varying amounts. An example of a system like this is the BorgWarner i-Trac(TM) system. Note the Haldex based car list was created from the list on Haldex corporate web site: | Haldex Cars. Interestingly a version of the BorgWarner ITM3e system is used on 2006 and up Porsche 911TT's. These Borg Warner systems were for runner of the populer Volkswagen DSG gearbox.
Off Road Drive (no center diff) (aka part time 4wd)
Jeep Wrangler (Rubicon has a locking front as well as rear)
Chevrolet Tahoe Z71, Suburban Z71, Silverado Z71, Colorado Z71
GMC Yukon Z71, Sierra Z71
Dodge Powerwagon (a ram version with front and rear lockers)
Nissan Titan, Xterra
Toyota FJ cruser (auto trans models), Tacoma
Chevrolet Trailblazer and GMC Envoy
Ford F series FX4, Explorer, Expedition, Sport Trac (all control trac 1)
Lincoln Mark LT
Dodge Ram, Dakota
Toyota Tundra TRD
Jeep Liberty (Comand-Trac)
Ford Ranger (torsen rear diff)
Jeep Cherokee (Quadra-Trac 2)
Dodge Nitro (Quadra-Trac 2)
Lincoln Navigator (Has slip sensing which can auto place into 4hi)
Infiniti QX56 (All-mode 4wd) Autoengages 4wd with slip
Nissan Armada, Pathfinder (All-mode 4wd) Autoengages 4wd with slip
Note Off Road Drive systems may not be driven in 4wd mode on dry pavement as damage to the transfer case will occur
Drivetrain Driveability Matrix
Drive train/Dry road/On road + light snow and rain with dry spots/Deep Snow/ Off road capable
Front Wheel Drive/Yes/Yes/Not well/No
Part-time 4WD/Only in RWD mode/Only in RWD mode/Yes/Yes
Full-time 4WD/Yes/Yes/Yes/Yes