You can have 999 on your armor by using the replenish armor cheat (hold L1, L2, R1, and R2 and press select) then press select again. This will give you 999 on your armor.
Auto-Target
Press START to pause the game, then hold L1 + L2 and press SELECT. The code must be entered after loading a game or a new stage.
Half-Life reference
During the first time you are on Voyager after the simulation, when you shut down the power core Belanna will say to someone "Help me with the Resonance Cascade Core". A Resonance Cascade scenario happened on Half-Life.
More Stuff
Pause the Game and enter these codes, you should hear "Impressive".
Maximum Health
Simultaneously press and hold L1, L2, R1, R2 and Select.
Refill Ammo
Simultaneously press and hold R1, R2 and Select.
Stuff
Pause the game and enter these. If it worked you will hear a voice say "Impressive".
All Weapons
Simultaneously press and hold L1, L2, R1, R2, L3, Select.
God Mode
Simultaneously press and hold R1, R2, R3, L1, L2 and Select.
Maximum Armor
Simultaneously press and hold L1, R1 and Select.
Undead mode
Press Start to pause game play, then hold R1 + R2 + R3 + L1 + L2 + L3 + Select. A female voice will say "Impressive" to confirm correct code entry. The code must be re-enabled after loading or upon entering new areas.