There are just a few for this game available. These first two codes have to be entered during the intro movie before the main game title page comes up.
Push up, up, left, left, right, right, circle, circle, down to get double health.
Up, down, left, right, triangle, up, down, left, right, square will add a silencer to your gun.
This last code only works after beating the game the first time. Again, this gets entered during the opening movie clip when the game starts up. You should see a green flash along the top and bottom if done correctly and to deactivate it just enter the code again.
Square, Triangle, Right, Circle, Left, Left will activate some extra visual effects.
That is pretty much it other than descriptions of how to unlock items.