Many people have the problem of one side being bigger than the other. Most commonly it's the arms but many people also suffer from uneven pecs, lats, delts, etc. This is usually from growing up using your dominant side to do all your lifting and moving in every day life. When I first started lifting my right arm was 3/4" bigger than the left and my left lat was barely there when I had decent size on my right. Here's some things that I used to help give myself aswell as others a more symmetrical body.
Use dumbells for all mass movements. That includes; all press movements and all curling movements. You can still use barbell movements but try to keep them to a minimum. When doing anything with cables try to keep it so each side is isolated separately (flies, one armed cable curls, one armed tricep pushdowns, etc.). By doing this it keeps the strong side from being able to cheat for the left side.
Here's a list of some basic exercises and how to use dumbbells and cables to even muscles up.
When selecting the weight to lift select by the strength of your weak side. You want to get 8-10 good reps with your weak side. The strong side will still be getting worked but more at a maintenance level which should allow the weak side to catch up.
Chest movements - let your weak side determine weight.
-flat bench
-incline bench
-decline bench
-flies
Arm movements - start with your weak side and only do as many on your strong side as your weak side could do. Concentrate on good form with heavy weight.
-alternating db curls
-alternating hammer curls
-one armed preacher curls
-one armed cable curls
-one armed tricep pushdowns
-one armed overhead extensions
-one armed tricep pull backs
Back movements - again start with the weak side and only do as many sets with the strong side as the weak side could do.
-wide grip pullups extending arms to lockout on each rep
-one armed seated rows
-one armed lat pull downs
-db lawnmower pulls
-deadlifts should still be done
Shoulders - select weight according to weak side.
-db shoulder press
-rear delt raises
-front raises
-Arnold press
-db shrugs
Most peoples legs are even. If not they should even out fairly easily with a good heavy leg routine. If the calves are uneven then just add in one leg calf raises.
The excercises I listed above are just general excercises to choose from. There are others out there but those will give you a good idea of what needs to be done. That also is not a routine. Just a list of excercises to choose from that will build a routine for those that want to even up certain muscles. After the muscles have evened up then a new routine should be started to better optimize growth. The information above is from personal experience. If anyone disagrees with any of it or would like to add to it eith what has worked for them feel free to do so.
If anyone needs help with a routine feel free to post it in here or start your own thread and I'll be happy to help.