During the game press the tilde (~) key to open the console window, where you can type in the following codes:

God Mode - GOD
Fly Mode - FLY
Fly Mode Off - WALK
No Clipping - GHOST
Add More Ammo - ALLAMMO
Get Rid of Artifacts - FLUSH
Open Map - OPEN followed by the map name
Third Person Perspective - BEHIND VIEW 1
Back to First Person Perspective - BEHIND VIEW 0