Invincibility:
During any mission, press White, L, Black, R, A.

Unlimited Ammo:
During any mission, press White, L, Black, R, Y.

Unlimited Time in Kaos Mode:
During any mission, press White, L, Black, R, B.

All weapons:
During any mission, press White, White, R, R, A.

Complete Current Mission
During any mission, press Left, Left, Left, Left, Y.

Mission select
During any mission, press White, L, L, L, White, A

Punches Decapitate:
During any mission, press White, L, Black, R, X.

Big Player:
During any mission, press Black, R, White, L, Y.

Little Player:
During any mission, press Black, R, White, L, A.

Normal Sized Player:
During any mission, press Black, R, White, L, B.

Looting on the Rise:
During any mission, press Black, White, R, L, Y.

Unlock Spanky:
In Kaos Mode, press Right, Right, Right, Right, Y during play. Or, beat the Mall level in Revolution Mode to make him availabe in Kaos Mode.

Unlock Freak:
In Kaos Mode, press Right, Right, Right, Right, B during play. Or, beat the Chinatown level in Revolution Mode to make him available in Kaos Mode.

Unlock Bull:
In Kaos Mode, press Right, Right, Right, Right, A during play. Or, beat the East Side level in Revolution Mode to make him available in Kaos Mode.

Unlock Chinatown Stage:
Get the #1 High score in Capitol City Mall in Kaos mode.

Unlock East Side Stage:
Get the #1 High score in Chinatown in Kaos mode.

Corporation Central level Score:
Get 100,000 points in the East Side level in kaos mode.

Last Clone Standing levels:
Complete the three minute and five minute versions of a level to unlock the Last Clone Standing version of that map in kaos mode.

Defeating ATO Guards:
While in revolution mode, drop your weapon before moving past an ATO guard. They will ignore you if you are not causing problems and have no weapon. You can usually find another weapon replacement later.

Use a Head as a Weapon:
Enable the "Punches decapitate" code, then punch and kill someone. Pick up his or her head to use it as a weapon.

Destroy Buildings:
Directly face a building window and throw/fire an explosive into the window to destroy the building. Any explosive (rocket launchers, grenade launchers, and molotov cocktails) will do the trick.

Thanks to Revolution readers Zak Maynard, Bucky Lament and Revrev!