Welcome to Gaia! ::

First, are all the cats spayed or neutered? Intact cats are more likely to be territorial and aggressive.

Also, make sure that the cats have their own litterboxes and their own dishes and separate areas to eat, so that there is less reason to get territorial. Give them their own space - keep the cats in separate rooms if need be, and let them out for supervised introductions.

I haven't tried any of these myself, as my girls get along just fine, but here's a few things you might try:

Feliway
http://www.feliway.uk.com/
It's a synthetic cat facial pheromone that is supposed to provide a calming and destressing effect. It helps with inappropriate urine marking, stress and aggression.

Flower essences
Just a couple of random pages:
http://www.catfaeries.com/essences.html
http://www.rainbowcrystal.com/bach/floweran.html
They're supposed to provide a calming effect that restores emotional balance. There's difference essences for different situations.

Clicker training
http://www.clickertraining.com/
Clicker training can be effective in resolving a wide range of undesirable behaviour and can help calm down cats that are aggressive. By marking and rewarding good behaviour the cat learns what is desirable and undesirable behaviours are reduced because they are not rewarding.

Anti-anxiety medication
If there is a very high level of stress in the household and they're not integrating you might consider talking to your vet about getting some anti-anxiety medications for the cats. Your vet would be the best to advise you on what would be most appropriate.

Personally, I would never again give my cat any sort of anti deppresent medication, I have in the past (because he is at times very aggressive) but I found it changed his personality greatly. He was no longer the cat I fell in love with. I took him off and he was back to normal. Mine was neutered and still very aggressive towards my other female cat, she would fight back but then they both got hurt. So Here was my solution, it worked for me and it's dirt cheep, you might want to try it. I got it from the book "The Psychology of Cats".

#1) Buy yourself a cheep little water gun, you know the kind kids play with.
#2) Always keep it filled with water. And nearby.
#3) Anytime You see the aggressive cat even attempting to make a move on the less dominant one shoot him in the nose with the water gun. (it has to be on the nose as the water gun will not penetrate his fur on the first shot and it will have no effect)
#4)If you want them to be friends than just do the above steps when they are about to get aggressive or are fighting. if you do it everytime they get near each other they will end up avoiding each other.

This method is similar to turning a hose on 2 dogs that are fighting, it distracts them from each other. Be warned though you will not see an improvement overnight, this must be repeated over and over again. It may take several months before you see a result. Although I do think it's better than giving your cat mind altering drugs. Some things should not be given a quick fix, drugs should really be a last resort, not to mention it could get very expensive and also the tables might turn. Your docile cat may see that the aggressor is no longer "large and in charge" and the docile one may turn on his former aggressor.

You may want to choose something other than a water gun so the basic psychology is just to distract the aggressive cat, you can use loud noises like pots and pans banging together, really anything to distract them. Over time they will learn that the act of aggression towards each other will result in something unpleasant to them (like a shot of water on the nose).