Go to the "Extras" option at the main menu and pick "Cheats" -> "Unlock Skin". Then, enter one of the following codes to activate the corresponding effect:
WOODCHIPPER: get 1990 skin
CEILINGFAN: get 2000 skin
NAILGUN: get 1980 skin
Now you can get a gold medal rank on the indicated level to unlock the corresponding cheat in an apartment sub-menu:
Crazy Deaths: Lucky's Bar level
Gun Splitters: Crack House
Unlimited Ammo: Pier 74 Revisited
Invulnerability: All levels
No Reloads: Stark Towers
One Shot Kills: Ryker's Island
Unlimited Slaughter: Takagi Building