I am surprised at the failing to mention melee, though Gnome touched on it. When you get swarmed there is bound to be one or 12 of those infected SOBs that gets through your fire and are in your face.

When you are being attached the game alerts you to where the attackers are with red arrows around your screen. I found that turning to where the most infected are and rifle butt (right click) then shoot, turn to where the most are butt and shoot, rinse, repeat until you can backpedal away from the group. Mostly effective with the shotgun. If you have the oozy/assault or are down to the pistols, you will need to butt and then spray at the heads.

I've gotten out of some hairy situations with this. I imagine using the military tactics Fatcat laid out you should not get into this situation, but useful if you do.

Also a couple things I have found that not all players have figured out:
-The infected are drawn to light. Use your flashlight sparingly.
-Peek around corners, there might be a boomer. If there is, try to sidestep out and shoot him. If he is to close you will have to push him away first (right click) and then shoot him or he will explode on you and you will get green view and violins.
-Witches are bitches. You watched the intro sequence right? Leave her alone! Don't put your flashlight on her and DO NOT shoot at or around her. Most of the places she has popped up, you can walk around her. Try to get as far away as possible so if other infected show you you don't end up spookin' her.
-Pipe bombs are very useful in open areas (streets...) but not so much in close quarters.
-Molotovs help put a barrier between you and a hoard. Use them to cut entrances for the infected. Don't throw them at your teammates! It doesn't help.
-Don't shoot the cars! Some set off an alarms and attract hoards of infected.
-The minigun overheats and stops firing. If you just mow down without pausing, you will not make it through the entire hoard. Make sure to watch the players back that is using the minigun and make sure to kill any smokers as fast as possible. Players on the minigun cant move and are often a target for a smoker to swipe them right away from that bad boy.
-Tanks are a pain in the ass, but can be killed. I've ye to find an effective way to get rid of him.

Lastly, Valve put up some wallpapers up on the L4D blog. Download them and use one for each of your screens in your quad monitor setup.

Enjoy!