At the 'Password' screen enter the following passwords to unlock the corresponding levels.

Unlock Down The Tubes Level:
Enter Paint, Faucet, TV, Faucet, Cone, TV.

Unlock Snot a Problem Level:
Enter Cow, Kettle, TV, Cow, Kettle, Paint.

Unlock Level 5:
Enter TV, TV, Cow, Can, Cow, Hammer.

Unlock Buttville Level:
Enter Can, TV, TV, Kettle, Cone, Cow.

Unlock Andy Asteroids Level:
Enter Cone, Faucet, Cow, Cow, Cow, TV.

Unlock Peter Puppy Level:
Enter Cow, TV, Kettle, Cow, Kettle, Paint.

Unlock Pod Races Level:
Enter Cone, Kettle, Cone, Cow, Cow, Kettle.

Unlock What The Heck Level:
Enter Kettle, Faucet, Cow, Faucet, Cow, Faucet.

Unlock Darkness:
Enter Kettle, Paint, Can, Faucet, Paint, Faucet.

Enter the following codes during gameplay to unlock the corresponding effect.

Unlock 1st 5 Levels in File Menu:
SLAUGHTERHOUSE

Unlock Infinite Continues:
ONANDONANDON

Unlock +1 Life:
ITSAWONDERFUL

Unlock +1000 Ammo:
POPQUIZHOTSHOT

Unlock Pitfall Jim:
HATMAN

Unlock Afro Jim:
BLOATED

Unlock Arrow Head Jim:
SLUGFORABUTT

Unlock Credits 1:
IDDQD

IDKFA Credits 2

Unlock Four-Eyes Jim:
MALFORMED

Unlock Full Energy:
Noble

Unlock Groucho Jim:
PULSATING

Unlock Lips Jim:
SWEATY

Unlock Bug Jim:
PUSFILLED

Unlock Homing Missle:
Apollo13

Unlock Level Select:
Getthecheesetosickbay

Unlock Max Life and Energy:
Threemileisland

Unlock Red Jim:
FESTERING

Show Frame Rate:
Framerate:

Suicide:
Trustnoone