With the '-console 1' command line parameter start the game and press '~' during game-play then enter any of the following codes. Commands will auto-complete as you type them. To see various console commands type any letter and you will be able to scroll up and down with the cursor keys to see the available options.
God Mode:
god

Get Most Items and Weapons:
impulse 101

Enemies Don't See You:
notarget

No Blood Loss When Using Disciplines:
freecasting

No Clipping Mode:
noclip

Exit to Desktop:
quit
Spawn Weapon (scroll up/down to choose):
give item_w_
Spawn Artifact/Book (scroll up/down to choose):
give item_p_

Spawn Item (scroll up/down to choose):
give item_g_

Spawn Clothing (scroll up/down to choose):
give item_a_

Spawn Money Item (scroll up/down to choose):
give item_m_
Give XP (#=amount):
giftxp #