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

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

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.

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