Unreal Tournament

Pause the gameplay and enter the following cheat codes:

Refill health
Press Down, Down, Down, Left, Up, Right.

Full ammo:
Press Left, Right, Left, Right, Right, Left.

Invincibility
Press Left, Left, Right, Right, Down, Up.

Level skip
Press Up, Down, Left, Right, Right, Left.

Unlock Single Player Challenge Mode
Finish Deathmatch, Domination and Capture the Flag modes
on any difficulty level.

Untested console cheats:
Note: You need a Dreamcast Mouse, and Keyboard.

Press "T", after hold the ESC key, then press any letter
on the Keyboard. Now type the following cheat codes:

allammo - 999 ammo for all guns
behindview 0 - returns you to regular view
behindview 1 - lets you see behind yourself
fly - lets you fly
ghost - Go throught walls
hidehud - Disable HUD
loaded - Random weapons
suicide - Commit suicide