The following codes are for SINGLE PLAYER ONLY. Press [TAB] to open the command line, then type:
Code - Result:
god - God Mode
loaded - All Weapons and Ammo
allammo - Full Ammo
allweapons - All Weapons
ghost - No Clipping Mode
fly - Fly Mode
walk - Disable Fly Mode
skipmatch - Win Current Match
teleport - Teleport to Crosshair
invisible x - Invisibile (x=true/false)
amphibious - Underwater Breathing
stat fps - Show FPS Stats
stat net - Show Network Status
behindview 1 - Third Person View
behindview 0 - First Person View

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

WARNING!: Always make a backup before overwriting game files.