Type in the following codes at the console (press ~ to enter the console). After typing in a code, press Enter, then exit the console.

AllAmmo		  Full ammunition
Fly		      Fly Mode
Ghost		  No clipping/Walk through walls
God		      God Mode
Invisible 1/0	Invisibility - 1 activates, 0 deactivates
Open XXXXXX	  Level Skip - Replace XXXXXX with any map name
Walk		  Turn off Ghost or Fly modes
behindview 1  External view   
behindview 0  Normal view   
flush         Reset bad textures on walls or creatures  
hideactors    Hide all monsters, weapons, and items   
showactors    Show all monsters, weapons, and items   
killall XXX   Kill selected monsters    
killpawns XXX Kill all monsters   
open          Level or server select
playersonly   Disable timer   
slomo         Set game speed (1 is normal)    
summon        Summon weapon, item, or monster1    
suicide       Suicide  
   

SUMMON KLINGONS.XXXXXXX
To summon weapons, items, or enemies, replace XXXXXX with any of the 
following names:	

AndorianBabe
AndorianCaptain
AndorianGrunt
AndorianThress
AntiGravBelt
AssaultDisruptor
AttackDroid
AttackDroid1
AttackDroid2
Bat'leth
B'Etor
BigBoxAlien
BigBoxKlingon
BirdOfPrey
BireQT
BloodWine
CargoShip
CipherKey
CombatArmor
CombatGoggles
Communicator
D'kTagh
DilithiumCell
DWCaptain
DWGrunt
FlashGrenade
Gagh
GeneticKey
GrenadeLauncher
HERocketCase
HGCaptain
HGGrunt
IceFish
Korek
Krax
Lethian
Lursa
MagBoots
MediKit
MediPatch
Nausican
PalmKey
ParticleCannon
PasscardKey
RetinalKey
Ro'peD
RPBeast
RPCaptain
RPGrunt
SithHar
SpinClaw
StealthSuit
TallBoxAlien
TallBoxKlingon
TarChop
Targ
Tricorder
VacSuit
Warden

Thanks to Revolution readers Duk'lah B'en-olin ar'g!