Below are the cheats for various versions/levels:

Version 1.4 cheats:

Player 1: Hit F1 4 times = invincible.  (F1 again to stop).   
          Hit F2 2 times for a missle.

Player 2: Hit F5 4 times = invincible.  (F1 again to stop).   
          Hit F6 2 times for a missle.

Hit F9 for a screen capture (filename: COPYxxx.PCX)

==> Back to the Forest:
 Infinite lives: TOAD
Infinite Energy: HARDNUT

==> Desert Raid:
 Infinite lives: NUTS
Infinite Energy: POWWOW
  Infinite Ammo: INSANE

==> Lost in Space:
 Infinite lives: SKUNNY
Infinite Energy: HUMHA
  Infinite Ammo: KICKBUT

==> Save our Pizzas:
 Infinite lives: TOGA
Infinite Energy: CHEESE
  Infinite Ammo: SLING

==> Wild West:
 Infinite lives: OHYEAH
Infinite Energy: BURGER
  Infinite Ammo: WATER