Press [Tab] key to bring down the console and then type:

god - God mode
fly - Fly mode
walk - Walk mode
ghost - Walk through walls
loaded - All weapons and ammo
allammo - All ammo
allweapons - All weapons
skipmatch - Win current Match
slomo # - Sets the speed of the game (1 = normal, -1 = cool, 0.5 = slow, 2 = fast, etc...)
behindview 1 - Changes to third person View (0 to turn it off)
invisible 1 - Invisibility (0 to turn it off)
amphibious - Underwater breathing
summon <item> - Summons an item (see the list below)
stat fps - Display FPS
stat net - Display network status
stat all - Show all stats
jumpmatch # - Jump to a specific match on the ladder
changesize #.# - Change the size of the player
lockcamera - Toggle locking the camera
freecamera 1 - Unlink the camera rotation from the actor (0 to turn it off)

viewself 1 - Reset the camera to view the player (0 to turn it off)
viewbot - Cycle the camera Through the viewable Bots
viewflag - View the actor currently carrying the Flag
viewplayer <player> - Change the camera view to the Given Player
viewactor <actor> - Change the camera view to the given actor
viewclass <class> - Change the camera view to the given class
killviewedactor - Kill the actor the camera is currently Viewing
avatar <class> - Possess a pawn of the given class
summon <object> - Summon the given object
setcameradist #.# - Set the distance of the camera
setgravity #.# - Change the gravity
setjumpz #.# - Change the jump height
setspeed #.# - Multiply the player water and ground speed with the number given
listdynamicactors - Output all dynamic actors to the log file
freezeframe #.# - Pause the game for the given duration
setfogr #.# - Set red color of fog
setfogg #.# - Set green color of fog
setfogb #.# - Set blue color of fog
setflash #.# - Set the duration a screen flash takes to fade away
causeevent <event> - Trigger the given event
logscriptedsequences - Toggle logging for all scripted sequences
killpawns - Kill all actor pawns except the player
killall <class> - Kill everything of the given class
killallpawns <class> - Kill all pawns of the given class
playersonly - Toggle the level to a players only level
cheatview <actor> - Toggle the view to the given actor
writetolog - Write the string "NOW!" to the game log
reviewjumpspots <option> - Test jumping where option is transloc, jump, combo or lowgrav.
add bots # - Add indicated number of bots
suicide - Commit suicide
open <IP address or map name> - Connect to server or load map
stat none - Disable statistics display
stat all - Display all statistics
stat game - Display game statistics
killbots - Kill all bots
exit or quit - Quit the game
disconnect - Quit the server you are connected to
demorec <filename> - Record demo
brightness # - Set brightness
contrast # - Set contrast
fov # - Set FOV
gamma # - Set gamma
cdtrack # - Set CD track music track
musicorder # - Set song intensity (0-3)
pausesounds - Silence
unpausesounds - Resume sounds
stopdemo - Stop recording current demo
togglefullscreen - Toggle full screen and windowed mode
confighash - View configuration information
mem stat - View memory status

Item List: xeffects.tankvictim, xweapons.ioncannon, xeffects.newioneffect, xeffects.shark, xweapons.redeemeer projectile, xweapons.grenade, vehicles.bulldog

Map List: BR-Anubis, BR-Bifrost, BR-Disclosure, BR-IceFields, BR-Skyline, BR-Slaughterhouse, BR-TwinTombs, CTF-Chrome, CTF-Citadel, CTF-December, CTF-Face3, CTF-Geothermal, CTF-Lostfaith, CTF-Magma, CTF-Maul, CTF-Orbital2, DM-Antalus, DM-Asbestos, DM-Compressed, DM-Curse3, DM-Flux2, DM-Gael, DM-Inferno, DM-Insidious, DM-Leviathan, DM-Oceanic, DM-Phobos2, DM-Plunge, DM-Serpentine, DM-TokaraForest, DM-TrainingDay, DOM-Core, DOM-OutRigger, DOM-Ruination, DOM-ScorchedEarth, DOM-SepukkuGorge, DOM-Suntemple. Plus, there is a vehicle demo map for mod makers to use as a demo of how to implement vehicles. It is appropriately called: VehicleDemo.ut2

Adrenaline Combos
When you get 100 adrenaline points, you can activate a combo (aka special move). They expire after a short time and then you have to gather more adrenaline to do another one. If you have an inverse mouse set, you may need to try reversing the order of the commands, ie - instead of Fwd, Fwd, Back, Back try Back, Back, Fwd, Fwd:

Berserk - Forward, Forward, Back, Back
Health Booster - Back, Back, Back, Back
Invisibility - Left, Left, Right, Right
Speed Booster - Forward, Forward, Forward, Forward

Double jump - Jump, then jump again at the top of the first jump.
Double dodge - Double tap Strafe Left or Right, Forward or Backward.
Dodge Jump - Perform dodge, then jump before you land.
Wall Jump - Run along a wall and jump into it. Press jump and opposite strafe when you hit the wall.
Lift Jump - When very close to the top of a lift, jump and you'll get more speed.
Shield Jump - Charge up the primary fire mode of the shield gun, look towards the floor and release. After the kickback, jump again to pull off the shield jump.

Double Dodge and Double Jump can be particularly useful for climbing rocks and slanted walls that might otherwise seem to steep. Simply try double dodging or double jumping forward as you face upwards against the wall or rocks in front of you. You can double jump to get to high areas and jump over large holes and gaps that might otherwise be too long for a single jump to get past. For example, in the Bombing Run game, when you deliver the ball into the ring, you must Double Jump in order to reach the ring barrier and get 7 points. You can shoot it into the ring for 3 points if you wish to score lower but not have to respawn.

Unlock All Skins
Open the UT2003.ini file (with notepad) from the System directory of your game folder and add the following lines to the end:

[XInterface.Tab_PlayerSettings]
bUnlocked=True
bTDMUnlocked=True
bDOMUnlocked=True
bCTFUnlocked=True
bBRUnlocked=True

Get the Redemeer and the Ion Gun in the demo
Edit the user.ini in the ut2003 system folder and set the following line:

Z=set XWeapons.Rocketlauncher FireModeClass class'XWeapons.RedeemerFire' | set XWeapons.RedeemerFire AmmoClass Class'XWeapons.RocketAmmo'

or to use the Guided Redeemer, use:

Z=set XWeapons.Rocketlauncher FireModeClass class'XWeapons.RedeemerFire' | set XWeapons.RedeemerGuidedFire AmmoClass Class'XWeapons.RocketAmmo'

or use this to get Redeemer's Gun model, too:

Z=set XWeapons.RocketLauncherPickup PickupMessage You got the Redeemer | set XWeapons.RocketLauncher FireModeClass Class'XWeapons.RedeemerFire' | set XWeapons.RocketLauncherPickup StaticMesh StaticMesh'WeaponStaticMesh.RedeemerPickup' | set XWeapons.RedeemerFire AmmoClass Class'XWeapons.RocketAmmo' | set XWeapons.RocketLauncher Mesh SkeletalMesh'Weapons.Redeemer_1st' | set XWeapons.RocketLauncher AttachmentClass Class'XWeapons.RedeemerAttachment'

'Z' being a random key that's not assigned. After that you can use the rocket launcher to shoot Redeemer ammo, you just have to hit whatever key you assigned it to before using the weapon. Enter ** summon xweapons.ionpainter ** during the game and - et voila - you can mark a spot to be hit by the satellite weapon. (NOTE: I don't know if you have to include the asterisks or not, I'm not at home , thus couldn't try it out myself yet.)

HINTS
Hidden vehicle demo map
A hidden vehicle demo map can be found in the "\ut2003\maps" folder. Play it and hit the switch on the ground. A giant car that shoots rockets will fall from the sky. Note: Type use at the console window when standing next to the car. You will be able to drive it.

Hidden announcer message
In the video options menu, turn every option to its highest value. The announcer will say "Holy shit".

Phrase on Redeemer Rocket
To see the phrase on a Redeemer Rocket, get one, fire the rocket, and enable the playersonly. This will freeze the rocket. Walk up to the rocket and you can read the phrase "Swallow This!" on it. You do not have to worry about getting too close to it. Since the Redeemer Rocket is something that you fired, it cannot harm you while airborne, unless it hits a wall or person close to you.