Press 'Start' during gameplay to pause the game and then enter the following codes to enable their corresponding effect.

Unlock ALL Weapons/Refill Ammo:
Press L, R, Left, R, L, Right, L, R, L, R, Left, R.

Unlock Level Select:
Press Left, Down, L, L, Up, Down, R, Right, Up, Right, L, R.

Unlock Infinite Health:
Press L, L, Left, R, Right, R, Up, Down, Left, R, Right, L.

When you beat the following missions the corresponding Multiplayer map will become unlocked.

Unlock Single Pak 3 Multiplayer Map:
Beat the King of the Chameleon (Mission 1).

Unlock Single Pak 4 Multiplayer Map:
Beat Saurian General (Mission 2).

Unlock Castle 3 Multiplayer Map:
Beat the Abominable Snow Turtle (Mission 3).

Unlock Wasteland 1 Multiplayer Map:
Beat the Giant Mech Turtle (Mission 4).

Unlock Wasteland 2 Multiplayer Map:
Beat Dark 'Devan' Shell(Mission 5).

Easy Serris

To beat Serris go to the very right of the room and climb to the top of the
ladder and fire from there (he will not be able to get you).

Blast ammo onto the wall and try and find the richochet gun on the second mission. Blast ammo against a wall and follow through to the next wall dodging as you go.

1000 Dollars
Pause the Game and press Up, Down, Up, Down, Left, Right, L, R, L, R, R, L.
This can only be used one time per game.

5000 credits
Pause the Game and press Up, Right, Down, Left, L, L, Right, Left, R, R, L,
L.
This can only be used one time per game.

1000 credits
Pause the Game and press Up, Down, Up, Down, Left, Right, L, R, L, R, R, L.
This can only be used one time per game.

500 credits
Pause the Game and press Right, Left, Right, Left, L, R, Up, Up, R, R, L, L.
This can only be used one time per game.