All levels
To get access type SKIPPY on the enter cheats menu.
[Submitted by: Chris]

NOTE: these cheats won't work unless you enable them on the cheats menu.

Code
Type in on the cheats menu GOLDENPIE this will give you all weapons, unlimited ammo, invincibility, etc.
[Submitted by: Chris]

Codes
Pause the game and select the "Cheats" option on the menu. then enter one of the following codes :

GODLY - God Mode
SORTED - Infinite ammunition
HIDEANDSEEK - Invisibility
SKETCHY - Wire frame mode
SONIC - Run faster
PNTNNP - Second level access
CLAW - Egypt Fodder in multi-player mode
GODDESS - Female Trooper in multi-player mode
UGLY - Hive Fodder in multi-player mode
LEGGY - Hive Guard in multi-player mode
RUBBER - Volcano Guard in multi-player mode
[Submitted by: Sn@ilB8er]

All Weapons
Pause the game and select the "Cheats" option on the menu. Select "Enter Cheat", then enter "LOADED" .