-THE HUNTER:
If the Hunter is outside pounce range, open fire. Head-shots are particularly effective.
Otherwise, throw a melee attack just as the Hunter starts his pounce. This will knock it back, stunning it, and you can finish it off with your choice of two additional melee attacks or weapon fire.
If you manage to stop the Hunter mid-pounce, keep hitting him as this keeps him from doing anything. Definitely use this tactic when he is close to other Survivors to avoid friendly-fire.
Once a Survivor is pounced:

You will receive a visual notification that a fellow Survivor has been pounced on.
Shoot at the Hunter if you are far away and melee it if you are close to it, friendly fire does not hurt a player while they are pinned down (although once the Hunter is off, they are then vulnerable to such).
If too far away and there is no Survivor closer than you are, go ahead and shoot.
A head shot with a shotgun or a sniper-type rifle will kill a Hunter in one shot.
Don't let walls stop you. If the only way to save a fellow Survivor is to shoot through a wall or ceiling, do it. The hunting rifle is particularly effective at this, although even pistols can punch through.

-HUNTER ON INFECTED TEAM:
The Hunter's job is similar to that of a wolf. The Hunter excels at picking off any stragglers or lone Survivors in any team. A Hunter's pounce pins a Survivor, leaving the victim completely helpless, until the other Survivors melee the Hunter or effectively kill it. Otherwise, without another Survivor's intervention, the pinned Survivor will die.

Due to the fact that the majority of players travel together, Hunters will not commonly find stragglers. This behavior by the opposition means that Hunters generally cannot successfully initiate an attack and cannot expect to do serious damage. As a result, Hunters are best sent in during a Panic event, when Survivors are blinded by vomit or bile, or any other cases in which the Survivors' attention is focused elsewhere. This lack of opportunity means that Hunters must stay stealthy and hidden until opportunities present themselves.

In order to pounce, Hunters must remain crouched for a limited time. However, being crouched causes the Hunter to growl loudly, broadcasting his arrival on the scene to any nearby Survivors. Also, whenever Survivors are in view of any of the Special Infected, the Special Infected will emit auditory sounds, such as aggressive growling in the Hunter's case. To avoid this, Hunters should remain standing and out of sight until they are ready to attack.

One of the Hunter's best attacks, that is infrequently used, is a straight melee attack by the Hunter in a standing position. This attack can deal a large amount of damage in a short amount of time. It is less noticeable than the pounce attack allowing a slightly longer time to deal damage. This attack should be used if the group is bunched together facing in the opposite direction or during the confusion of a boomer attack. An advantage to this method is that the Hunter does not emit a scream. This allows the player to be stealthier and easily mistaken as one of the Common Infected. Another idea is to use this attack to quickly incapacitate a Survivor in the red; one or two hits and they'll be on the ground.

Another effective tactic is to continuously wall jump. Hunters emit a distinctive scream during flight. An experienced wall-jumper can effectively distract a team, slowing them down, and buying the Infected players additional time, which may cause the Director to spawn a Horde. Also this can be used to tempt someone to stray away from the group to kill the Hunter. Then when he is alone, pounce on the lone Survivor.

A rather effective resort for the Hunter is to rely on his melee attack. Hunters can run up to survivors and claw at them without giving off many of the tell-tale signs of the presence of a Hunter.

-THE SMOKER: The Survivors Here are some basic tactics to help you deal with a Smoker:

When you or one of your teammates hear a Smoker, stay close together and watch out for each other. If any of the Survivors are alone and too far away from the group, they make an easy target for the Smoker. If the lone Survivor is grabbed, it's possible the other Survivors will not notice or will be too far away to help in time.
If a Smoker is on a roof and has grabbed a teammate then do not waste your time killing the Smoker and just melee the Survivor. The Survivor will automatically be freed.
When a Smoker grabs you, you will have a few seconds to free yourself by shooting the Smoker. If you are not currently holding a gun (or have started reloading) at the time your are grabbed, you will probably not have time to save yourself.
If a Smoker grabs you while you're throwing a pipe bomb, and no other Survivors are around or notice you, throw the pipe bomb at the Smoker. The resulting explosion will free you from the Smoker's grasp.

-SMOKER ON INFECTED TEAM:
Your job is to ensnare a Survivor and separate them from their friends.

Your "tongue" attack leaves Survivors helpless after about two seconds of being ensnared. A seasoned Survivor with quick reactions can break himself free from your "tongue" and kill you most of the time. However, a well placed attack in the midst of a horde rush can spell disaster for the Survivors if not dealt with properly. Always look to be either up high or in a dark place as the darker or higher you are the less chance the Survivors will be able to shoot at you.

An experienced Survivor can recognize the wheezing and coughing of a Smoker and will be alert and ready to save his teammates from being snared. If you want to be a real burden to the Survivors, stand in front of or behind an "alarmed car". If you stand by the car and pull a Survivor over to you, the other Survivors will have no other choice but to try and save the person you snared. Generally, instinct will override common sense, and the Survivors will open fire, inevitably activating the car's alarm. Any stray bullets that impact the car will set the alarm off, attracting the horde, and buying you more time to attack. This tactic also works similarly well with the Witch.

Good Smoker players excel at using the environment against the Survivors - this can be by dragging them down from a higher part of a level as the fall will do a lot of damage, in some places it will cause death. If the lower area is just where the Survivors have had to climb up from, the Smoker's target will have to climb back up to the higher area leaving them away from the group and slowing down the team. Smokers can also drag Survivors through fire that again causes large amounts of damage while either forcing another Survivor to run through the fire to release their constricted teammate or wait until the fire dies down while the Smoker does damage to the constricted Survivor.

-THE TANK: The Survivors General Tank tactics:

Keep your distance from the Tank if at all possible. You can't expect to survive a one on one with a Tank unless you have a decent amount of health and are an experienced player, but having to go one on one with Tanks should rarely happen. Take advantage of your surroundings by climbing up and jumping off of ledges, as the Tank leaves itself open whenever it has to climb.
Set the Tank on fire using either a Molotov or a gas can. As of January 23, the damage the Tank received from fire is now reflected in the Tank's health. This will reduce the amount of time you require to engage and defeat the Tank.
Explosives such as pipe bombs, propane, or compressed air tanks are great for limited big damage, but not as effective as fire, which always does guaranteed damage over time to the Tank. The propane and compressed air tanks will also cause the Tank to reel back, like Infected after a melee attack.
Keep clear of cars, fallen trees, furniture, and dumpsters if the Tank is nearby. The Tank will definitely use big objects in the environment and try to punch these objects towards Survivors, which means an immediate incapacitation if said object hits a Survivor. Tanks will try to climb over burning barrels, though, setting them ablaze (usually only applies to AI-controlled Tanks, but might work with some similar smart players), so moving to put a barrel between you and him will usually have the same effect as a well placed Molotov. Make sure you run, because you do not want to be pounded on by a burning Tank.
Try to draw the Tank into a crouching only area. It can buy you a few seconds while the Tank tries to wiggle its way through the area. Good examples of crouching-only areas are ventilation grates and ambulances. Note, however, that this is no longer a viable tactic as of the latest update. Tanks now crawl at a much faster speed and will quickly overwhelm a team hiding in vents.
The Tank can climb, but it slows down. Take advantage of this as you can climb up and down much faster.
In closed quarters, all Survivors should concentrate all of their fire-power on the Tank. The Tank will pound on a Survivor or two, but everyone should make it out alive.
Another useful tactic is to get to an area immediately inaccessible to the Tank. The rafters of a building away from the area that allows one to climb up to it is effective, because the Tank wants to get to you by the shortest route.
Whenever a Tank takes damage, its forward momentum slows down (just like yours slows down when Infected punch you). If you can light him on fire, hit him with a minigun, and have the other three pump Shotguns into him, the Tank slows to a virtual standstill and is quickly dealt with.
Lacking a good situation, have one person focus on avoiding the Tank while the other three keep him as slow as possible. Make sure to communicate and let your bait know if he switches targets.
Explosions, such as pipe bombs, propane tanks, oxygen tanks, and Boomers can knock the Tank back and give you a good three or four seconds to fill him with bullets. It's even better if you can push him off high ground, forcing him to come back up.
On Expert difficulty, the Tank can incap a Survivor with one punch or his rock throw.