Unreal

Type in these cheats on the console.

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