Unreal
(GT Interactive)
Nothing's easy, especially Unreal. However, type the following codes into the console and hopefully things will become a lot more tolerable.

GOD	
Invulnerability

ALLAMMO	
Gives you 999 ammo on every gun

GOD	
Lets you fly

GHOST	
Enables you to walk through walls

WALK	
When flying or ghosting, this returns you to normal

SUMMON	
Enables you to summon an item. For example: 
SUMMON EIGHTBALL
SUMMON FLAKCANNON
SUMMON NALI
SUMMON SKAARJWARRIOR etc

PLAYERSONLY	
Freeze time. Press again to resume

OPEN MAPNAME	
Jump to any level. For example: 
OPEN DIG

BEHINDVIEW 1	
Gives you a third-person perspective camera angle

BEHINDVIEW 0	Restore first-person perspective