You have to enable the "Console" window ("Settings" -> "Game Options") and then press ~ and enter one of the following codes during the game:
God mode: god
No clipping mode: noclip
All weapons: wuss
All weapons and full ammunition: give all
Set health level: health [1-100]
First person view: cg_cameradist -45
Return to default view: cg_cameradist 128
Disable enemy AI: notarget
Frame rate display: fps [0 or 1]