Press ~ to bring up the console and enter the following codes:

God	Invincibility (not from falling)
Loaded	Get every weapon
allammo	999 ammo
Ghost	Go through walls and gain flight
Walk	Turns Ghost off
suicide	Kill yourself
FOV X	Change X to change Field of View, where default is 70
behindview 0	First Person View
behindview 1	Third Person View
Slomo X	Change X to increase / decrease time, where 0.2 is very slow.