Level Select:
At the menu press LS, RS + Up, then press A to start a new game. From the difficulty menu press LS, RS + Down and select a difficulty level. Press B during Begin Transmission display, then highlight a level and press A to start the game at that level with the appropiate inventory.

Invulnerability:
Use the frying pan as the current weapon. Press C, C to display weapon select option, then highlight the powerdrill and press A, RS. The words "KNEEL BEFORE ME!" will confirm this code.

To cancel invulnerability enter the Weapons Select menu and press A, RS.

Full Inventory:
Note: This only works on a standard controller.

While playing, press C + X to enter 3D map mode. Rotate the map on any axis. Press P, then LS + A + B + C while the map rotates back to the default view. Congratulations! You now have all the weapons and a jetpack.