Using cheat codes will flag you as a cheater until you exit back to the main menu (Campaign) or play a new level (Rampage). While you are flagged as a cheater, the following will happen:

• You cannot unlock achievements
  
• Progress you make toward locked achievements will not count
  
• Saving the game is disabled, including autosave (Campaign)
  
• You will receive a "Cheats Used" penalty equal to the amount of points you score (Rampage)
  
• High scores and grades will not be saved and you cannot unlock or view the ending until a score has been recorded for every level (Rampage)

Cheats only work in single player, they do not work in Co-op mod.

To activate cheats, go into a single player game. The cheats will not work in Co-op mod.

• Press T* to open the console
  
• Type the cheat code onto the console
  
• Press enter once you have typed the cheat code

If you got it right you'll hear the Dude make a comment.
*depends on which button you attributed to the 'Text chat' command in Options -> Controls -> System.

CHEATS LIST

General

Healthful - Maxes out health
BlockMyAss - Maxes out armor
CarryMore - Gives backpack

Weapons & god mode, sonic mode and opening the exit

AssaultAndBattery - Gives assault rifle
BlowMe - Gives balloon gun
Flamenstein - Gives flamethrower + ammo
TitanIII - Gives rocket launcher + ammo (normal and seeker)
Sternomat - Gives napalm launcher + ammo
PeaShooter - Gives pistol
JesseJames - Gives revolver + ammo
TheBestGun - Gives scattergun + ammo
Shellfest - Gives shotgun + ammo
CrotchBomb - Gives mines
LobItFar - Gives grenades
Rioter - Gives molotovs
Dynomyte - Gives dynamite
MagicMags - Gives infinite ammo for all currently-owned weapons
AlaMode - Toggles god mode
NoPlaceLikeOz - Opens exit
YoureTooSlow - Toggles sonic speed

General + Weapons & god mode, sonic mode and opening the exit

IAmSoLame - All of the above in one cheat

Miscellaneous

BegForThis - Wimpy enemies mode (one shot injures)
NoHUDPlease - Toggles HUD visibility
MyteaMouse - Shrinks all characters (toggle)
GimmeHead - Enlarges all characters' heads
WitchDoctor - Shrinks all characters' heads
YourHeadASplode - All enemies have their heads exploded on death
SuddenlyISee - Maxes out the camera view
TunnelVision - Drastically contracts camera view
ClowningAround - Turns all NPCs into clowns
RaisedInABarn - Turns all NPCs into dancing ostriches
LongArmOfTheLaw - Turns all bystanders into hostile police officers
MichaelBay - Turns all NPCs into rocketeers or grenadiers, and explodes all barrels on the map
Groovy - Toggles slow motion
TheQuick - Quick firing speed on all weapons

Special

mynameisnotimportant - Allows you to select The Antagonist (a.k.a. Not Important) from Hatred to play.
To do so, go to: Options -> Game menu -> click on the 'Coat Color' text to gain access to a 'Character select' screen.
The "Warnings" section in this guide does not apply to this specific code. In other words, you will be able to unlock achievements, etc. even if you type this code.
NOTE: to mimic Hatred even more, you can use the black and white effect which can be found under Options -> Video -> Effects and is called 'Just like that other game'. See an example here: https://www.youtube.com/watch?v=jWtYZGrwnBE