During game hit Tab to activate chat and type in the following to activate the cheat function:

god - god mode
allammo - 999 ammo for all guns
fly - lets you fly around
walk - use this to stop flying or ghosting
ghost - noclip through walls
playersonly - toggles time freeze on/off
open x - x is the name of the map to go to
flush - fixes wired graphics
behindview 1 - puts you in third-person view
behindview 0 - puts you back in first-person view
summon x - x is the name of whatever you want to the world (see below)

Summon Usage Items:
Translator
Amplifier
Dampener
Flare
Flashlight
Forcefield
Invisibility
Jumpboots
Scubagear
Searchlight
Nalifruitseed
Voicebox
Bandages
Nalihealingfruit
Superhealth
Health
Armor
Asbestossuit
Kevlarsuit
Anit-Toxinsuit

Summon Weapons:
Dispersionpistol
Automag
Stinger
Asmd
Eightball
Flakcannon
Razorjack
Gesbiorifle
Rifle
Minigun

Summon Creatures:
Nali
Skaarjwarrior