Various cheats
     While standing in the Helicarrier, use the d-pad to enter the code.

Cheat code:                            Effect:
Left, Left, Right, Up, Down, Down      Unlock Venom
Left, Down, Right, Right, Down, Left   Unlock New Goblin 


5,000 credits:
     Press  Up,  Up,  Down,  Down,  Left,  Right while standing on the
Helicarrier.