Enter the name ICHEAT. Now enter the follwing codes during play...

Refill health and fuel:
Down, Up, Right, A, B, Left, C-Right 

Fat Alien Legs:
Left, A, Right, Down. Makes aliens' legs bigger. 
Aliens must be present for this to work.

Freeze Aliens:
C-Down, C-Up, A, Right, C-Right, Down.

Unfreeze Aliens:
Z, Right, C-Right, Down

Mutant Cheat:
C-Down, Up, Z, Z, C-Right, Right. 
If a harvester is present, this will turn it into a mutant.

Serious Weapons:
C-Down, C-Up, Up, Z, Z, Left, C-Right. 
Increases weapon damage.

Surreal:
C-Down, Up, Right, Right, C-Right, A, Left. 
Makes the buildings bounce.

All Weapons:
A, Right, C-Down, C-Right, C-Up, A, Left.
Gives you all available weapons for that stage.

Smart Bombs:
A, C-up, C-up, Up, Left

Tall Adam:
B, A, C-Up, A, C-Up, A

Small Adam:
Down, C-Left, A, Right, Z

Kill Adam:
B, Left, C-Right, C-Right, Down 

Dancing Adam:
Down, Up, C-Up, Down, C-Right, C-Right

Evil Adam:
C-Left, C-Right, A, C-Down, C-Right, Left

All Artifacts:
Up, C-Down, C-Right, Z, Up, Left

Weak Boss:
Z, C-Right, C-Right, B, Left, C-Right 

GameShark Codes

Infinite Health		8104dcec0258

No Humans Killed	8104816a0000

Infinite Greece		81052b180098
			81052b1a967f

Extra Weapon And Ammo	810481380203
			8104813a0405
			8104813c0607
			8104813e090a
			810481468000
			810481488000
			8104814a8000
			8104814c8000
			8104814e8000
			810481528000
			810481548000

Thanks to Revolution readers Dustin Beyette, Viperboy, Ben Smith, Sufyan, Spud, Luke Phillips, Damian Germinario, Jon German, Joey, jackal_03, Chris Barrett, Short Stuff, Diamond Dog, Dan martin, and Jason "normanJ" Perrone!