Open the console with pressing [~] and type in:

Invuln God mode
GimmieGimmieGimmie All weapons available
NoClip No Walk / fly thru walls
NoTarget Invisible ON/OFF
Spectator Fly mode
ChaseCam Change camera angle (5 possible angels)
CyclePlayerClass Cycle through 4 player classes
NextArmor Refresh armor and health
PrevArmor Decrease armor
NextMonster Find next monster
PrevMonster Go to previous monster
RestartLevel Restart current level
Give [Item][Amount] Get items where [Item] might be

AirAmmo
AngelCharm
BeastClaws
Belt
BlackDiamond
BloodRose
Boots
ChaoHeart
CrystalBall
Draco
EarthAmmo
Fist
Galaxion
Gauntlets
Hawkeyes
Health
Key
MagmaAmmo
MagmaCannon
Pamphlet
PickupTrigger
Punisher
RageOrb
Scourge
SpiritLance
Stargaze
Thornblade
Ticket
Twister
WaterAmmo
Windblade
ZeroCannon