Slayer


 CHEAT MODE:

 Make your way to the end of the first stage ensuring you're on
 your very last life. Now blow up all of the enemy guns and the
 little green monster. Next crash into the machine it resided in.
 You will now start on level two with infinite lives.

 Another cheat says:
 Confront the end-of-level guardian with one life intact. Press 'D' then
 crash into its dead body. You'll now have infinite lives.