God Mode:
Pause the game and press Square, Circle, Left, Right, Circle, Square.

Max Ammo:
Pause the game and press Left, Right, Circle, Circle, Circle, Right, Left.

All Weapons:
Pause the game and press Left, Circle, Right, Square, Right, Left.

Level Skip:
Pause the game and press Up, Down, Left, Right, Right, Left, Circle.

All Characters:
Pause the game and then press Left, Left, Circle, Circle, Right, Left, Circle.

Level Select:
Save your game and go back to the main menu. Once you are at the resume game screen, highlight your saved game and press Up, Down, Down, Up, Left, Up, Right, Down.

Unlock Fatboy Mutator:
Go to the title screen and press Circle, Circle, Circle, Up, Down, Down, Up, Circle, Circle, Circle.

Unlock Stealth Mutator:
Go to the title screen and press Square, Square, Circle, Circle, Square, Square, Circle, Circle.

Unlock Big Head Mutator:
Go to the title screen and press Left, Right, Left, Right, Left, Right, Circle, Circle, Circle.

DREAMCAST:
Level skip:
Pause the game and press Up, Down, Left, Right, Right, Left.

Max ammo:
Pause the game and press Left, Right, Left, Right, Right, Left.

Refill health:
Pause the game and press Down, Down, Down, Left, Up, Right.

God mode:
Pause the game and press Left, Left, Right, Right, Down, Up.

Unlock SegaNet levels:
At the main menu, select the "Network" option, then choose the "Account" selection. This should switch to "Account 2". Use the following configuration:

Login name: admin@sega.net
Password: tenages (Seganet spelled backwards)
Area code: Type in your local three digit area code (Ex. 123)
Phone number: Leave blank (do not type in anything)
Primary DNS or DNS1 = 0.0.0.0
Secondary DNS or DNS2 = 0.0.0.0

Click on "Connect" and the game will ask if you want to save changes. Click on "Yes". You should now have access to all levels in deathmatch mode (if you already have completed the game or used the "Level skip" code). Go to practice mode to find two new levels that were previously locked. Note: Make sure you do this on "Account 2" or your online configuration will be overwritten -- and you will then have to set everything up again.