XBOX 360:

While in game, open the pause menu, hold the left shoulder button and enter the following code:
(single player only)

Third person view - X, B, X, B, Left Thumbstick, Left Thumbstick, Y, A, Y, A, Right Thumbstick, Right Thumbstick

All characters have big head - B, X, A, Y, Left Thumbstick, Y, A, X, B, Right Thumbstick

Characters are kill we only one shot - Left Thumbstick, Right Thumbstick, Left Thumbstick, Right Thumbstick, A, B, Left Thumbstick, Right Thumbstick, Left Thumbstick, Right Thumbstick, X, Y

Change bullet tracers color - Left Thumbstick, Left Thumbstick, A, Right Thumbstick, Right Thumbstick, B, Left Thumbstick, Left Thumbstick, X, Right Thumbstick, Right Thumbstick, Y

Increase ragdoll strength - A, A, B, B, X, X, Y, Y, A, B, X, Y

Multiplayer Tracers
You can use this to turn on tracers via xbox live so long as you are the host of the game. LS is Left Stick RS is Right stick

Cheat Effect
Press Start as host then hold LB, LS twice, A, RS twice, B, LS twice, X, RS twice, Y Turns on multiplayer tracers Red for Red team Blue for Blue

 Playstation 3:

Start a game and press START during gameplay, then hold down L2 while entering all codes.

One Hit Kills - L3, R3, L3, R3, X, O, L3, R3, L3, R3 Square, Triangle

Super Ragdoll (When you bullets hit enemies they go flying) - X, X, O, O, Square, Square, Triangle, Triangle, X, O, Square, Triangle

Big Head (Everyone has a big head) - O, Square, X, Triangle, L3, Triangle, X, Square, O, R3

3rd Person (Changes view to see your whole body) - Square, O, Square, O, L3, L3, Triangle, X, Triangle, X, R3, R3