Press [t] to go to the talk mode and enter

 hongkong   - infinite ammo, all weapons
 mpkfa      - God Mode
 keymaster  - all keys
 idaho      - All Weapons/Ammo
 capinmyass - Message: were you cheating?
 montana    - All Weapons/Ammo/Items
 griswold   - Full armour in for all three armour types
 edmark     - Kills you
 jojo       - Makes you trip out for a bit
 I wanna be like kevin - Message: Now you are just like Kevin
 satchel    - Gives you all the items
 onering    - Activates Invisibility
 mario      - Level warp
 kevorkian  - Kills you
 mcgee      - Light you on fire
 krueger    - Flame resistance
 cousteau   - 200 Health
 voorhees   - God Mode
 Lara Croft - Full weapons/ammo  (type It again to remove it all)
 sterno     - Temporary black-out
 Fork Broussard - Trippin Mode/No Items/No Weapons/No Ammo
 Eva Galli  - No Clippings Mode
 rate       - Shows screen rate in top right corner of screen
 goonies    - Full map
 spielberg  - Level Warp
 funky shoes - Lets you jump higher
 spork      - Gives you 200% health!
 tequila    - 2nd weapon
 bunz       - All weapons and you can carry two weapons

FunCheat:

 In E1L4 (the Dark Carnival), press the left-hand Warning sign in the
 Jo-Jo room. This opens up a room with a rather sad Duke Nukem hanging
 from the ceiling. Pushing Duke results in "Shake it, baby!".