At the title screen, select 'special'. From here select 'cheats'. Type the following for the listed effects.

Code - Effect

MRJUAREZ - Get Max Ammo
LRGARMS - Be Invincible
BUTTERUP - More Health
STRAPPED - Get All Weapons
QUITER - Skip Level
SLIZOMIZO - Slomotion Button
BOUNZMEN - Bouncy Men
PASSKEY - Level Warp All
DONOTSEL - Do not select
SCARYCLOWN - Get Clown Hero
SUPERJOHN - Get SuperJohn Hero
GNRLINFANTRY - Get GI John Hero
WEIGHTGAIN - Get Nara Hero
HAIRLOSS - Get Nikki Hero
BIGLIPS - Get Moseley Hero
JAILBREAK - Get Convict Hero
MRROBOTO - Get Robot Hero
IAMSODEAD - Get Zombie Hero
HISSSS - Get Lizard Hero
STYLIN - Dramatic Finish
CLUTZ - Clutter the Place Up
SPINACH - Get Max Damage
WIMP - Get Ending scene
STEELUP - Get Armor
CLUMSY - Get Rag doll
DIRECTOR - Unlock all movies
NINJA - Unlock all combos
SLUGGER - Get Baseball Bat
MAXIMUMHURT - Unlock Pain Arenas
FPSSTYLE - Get Free aim
SKETCHPAD - Unlock Concept Art