Ceriwis - [PS2] [Cheats] Area 51

Unlock Final Secret:
Grab all the other secrets to unlock the final secret (page 5). Well, d'uh.

Unlocking Secret Tapes:
Scanning all five databank items in each stage yields one secret. Completing the game yields one secret.

Unlockable: Alien Multi-player Avatar:
Beat Campaign mode on Hard difficulty setting to get alien Gray as a selectable multi-player avatar.

Hint: Upgrading The Alien Grenade:
In the stage Life or Death, there is a small key on a dead soldier in one of the lifts where Cole starts the stage. Grab it to open the small lockers in the stage. One of the small lockers in the first power conduit pit has a databank item that upgrades the alien grenade (it now homes in on enemies).

Unlock hard difficulty:
Finish the campaign mode in medium difficulty mode.

Unlock Alien avatar in multi-player mode:
Finish the campaign mode in hard difficulty mode.

Invincibility:
Pause game play, the press X, Circle, Circle, Triangle, Circle, X.

Start with a shotgun:
Pause game play, then press Triangle, Square, Triangle, Left, R1

Level skip:
Go to the main menu. Choose "Arcade", then aim at the black border
of the boxes with "Warp Ahead" or "Beginning" to begin farther
into the game than normal.

100% health all the time:
During gameplay, press R1, R2, L1, L2, Left, Up, Down

100% ammo all the time on any gun:
During gameplay, press R1, R2, R3, Left, Right, Down

XM-32 viper assault rifle:
During gameplay, press Left, Right, Down, Left, Right

God mode:
Pause game play, then press Up, Down, Up, Down, Left,
Right, Left, Right, Down

Source? Here it is.