Press 'Tab' to activate console and type these cheats:
GOD = Hmm...
FLY = Fly Mode.
GHOST = No Clipping.
WALK = Turn off Fly or Ghost Modes.
ALLAMMO = All Ammo. (Durr!!!)
SUMMON * = Summon anything. (eg. SUMMON SKAARJWARRIOR)
SLOMO * = Speed Modes. (SLOMO 0 for slow motion, SLOMO 9 for fast motion.)
SUICIDE = Kill Yourself. Exciting.
KILLALL = Kill All Enemies.
KILLPAWN * = Kill all of any enemy. (eg. KILLALL SKAARJWARRIOR)
OPEN * = Open Any Level. (eg. OPEN THESUNSPIRE)
PLAYERSONLY = Freeze time. (Excellent for target practise, the Razorjack comes into mind...)