When playing in singleplayer, press the tilde ( ` ) to lower the console and type in the following codes:

Code
/god
/give all
/noclip
/give [item_name]
/iamacheater
/iamamonkey
/g_gravity [number]
 Effect
God mode
All weapons and ammo
No clipping
Spawn item. See Item List below
Unlock all levels at skill 1
Unlock all levels at maximum skill
Change gravity. Default is 800

Item List

ammo
armor
battle suit
bfg10k
flight
gauntlet
grappling hook
grenade launcher
haste
health
invisibility
lightning gun
machinegun
medkit
personal teleporter
plasma gun
quad damage
railgun
regeneration
rocket launcher
shotgun