These only work for single player games (i.e., not Xbox Live). Press START and tap the D-Pad for:

Low Gravity - UP, RIGHT, RIGHT, UP, UP, LEFT

No Target Mode - LEFT, RIGHT, RIGHT, LEFT, UP, DOWN

IFF Glow - RIGHT, RIGHT, RIGHT, UP, DOWN, LEFT

Big Heads - UP, UP, LEFT, RIGHT, UP, DOWN

Player clipping - UP, DOWN, LEFT, RIGHT, DOWN, UP

Autoaim Toggle - DOWN, DOWN, DOWN, DOWN, UP, UP

Add 16,000 Credits - UP, UP, DOWN, DOWN, LEFT, RIGHT

Outfit Change - LEFT, RIGHT, DOWN, UP, LEFT, RIGHT