Activate the CONSOLE while playing then type any of the following commands to activate the corresponding cheat:

Code Effect

GOD God Mode
GODALL God Mode, All
GODTEAM God Mode for Team
GODTERRO God Mode for Terrorists
GODHOSTAGE God Mode for Hostages
GHOST No Clipping Mode
WALK Walk Mode
FULLAMMO Full Ammo
SGI Game Modes
RAINBOWSKILL MP Version
PAGO All Enemies Attack
RESETMEALL Reset All Cheats, Gameplay
KILLPAWNS Kill/Remove Opponents