Press [F12] to bring up the console, then type:

Cheat Code - Granted Wish

god - God Mode
mortal - God Mode Off
SetWoundedState - Walk as if Wounded
SetNormalState - Walk Normally
showfps - Show Frame Rate
getpainkillers - Get 8 PainKillers
c_addhealth ( 100 ) - Add 100 to Health
getallweapons - All Weapons
getinfiniteammo - Unlimited Ammo
noclip - No Clipping Mode
noclip_off - No Clipping Mode Off
getbullettime - Fill Up When Bullet-Time Runs Out