Press ~ to bring up the console and type in:

Beat the game on any difficulty to unlock the boss characters for play.

During a game on the map Asbestos go to the room with the lightning gun and up onto the raised balcony. If you stand exactly where the four adrenaline pills are located and look directly up you will see a tall narrow alcove in the ceiling against the wall. You need to shoot the ceiling in the alcove five times. You know you have shot it correctly as the text ''ownage'' will be displayed with each shot. With the fifth shot you need to run as fast as you can down the ramp to your right, past the string of five health vials and down the narrow corridor. When you reach the small room at the end of the corridow look up at the top part of the wall above the shock rifle, opposite the corridor you just came down. If you are quick enough a picture of Cliffy B with the text ''Miss me?'' below it will be revealed from behind a sliding wall. You have to run quickly as the wall will slide back to hide him approximately seven seconds after the fifth shot.

If you input your name as ''CliffyB'' (without the quote marks) and play an Instant Action game against bots, the bots will have the names of the Epic Games developers.

Warining: This cheat involves editing the contents of your ut2003.ini file. Improper editing could cause the game to stop functioning properly. It is best to create a backup file before you perform this cheat.
Add the following lines into your ut2003.ini file:

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