During game play press the '~' button and 'enter sv_cheats 1' then enter any of the following codes:

Unlock God Mode:
God Gives you

Unlock ALL Weapons and Ammo:
Impulse 101

Warp to Level:
Changelevel (Name of Level)

Invisible to Enemies:
Notarget

Sets your Health to 999:
Health 999

Walk through walls and Objects:
Noclip

During game play press the '~' (tilde) key to bring up the debug console and enter the following codes:

God Mode:
/superfuzz

Unlock 999 Health:
/health 999

Unlock All Weapons:
Wuss

Unlock Infinite Ammo:
Give infinite ammo

Unlock No Clip Mode:
/nocollision

Unlock No Target Mode:
/wallflower

Set Gravity Level:
Sv_gravity # (where # is the gravity level to set)

Spawn a Coin:
/spawn coin

Spawn Cookies:
/spawn cookies

Spawn Helicopter Gun:
/spawn heligun

Spawn a Dollar:
/spawn dollar

Spawn a Health Pack:
/spawn health

Spawn a Silver Key:
/spawn keyring

Spawn Papers with Evidence:
/spawn Evidence

Spawn Reactive Shields:
/spawn reactiveshields

Spawn an Assault Rifle:
/spawn assasultrifle

Spawn a Rocket Launcher:
/spawn rocketlauncher

Spawn the Silencer for the Magnum:
/spawn Silencer

Spawn a Yellow Security Card:
/spawn YellowCard

Spawn a Chain Gun (Weapon #5):
/spawn chaingun

Spawn a Green Security Card:
/spawn GreenCard

Spawn a Blue Security Card:
/spawn BlueCard

Spawn the Sniper Rifle:
/spawn sniperrifle

Spawn a bag of Money:
/spawn moneybag

Spawn a Light-Effect:
/spawn lensflare

Spawn the Shotgun:
/spawn shotgun

Spawn the Magnum:
/spawn Magnum

Spawn Rockets:
/spawn rockets