Enable the console by adding -console to your shortcut before starting the game. This can be done with steam easily by clicking properties in steam for the game, then SET LAUNCH OPTIONS and type it there.

Press the [~] key to bring up the console while in the game.

Cheat "silence" - Enables silent movement
Cheat "camouflage" - Appear disguised from the enemy
Cheat "invisible" - Guards can't see you
Cheat "disable ai" - Disables AI
setv cl_superman 1 - God Mode
setv c_noammo 1 - Infinite ammo
OpenLevels - Access all levels
PL_SavePos - Save your game
PL_RestorePos - Load your game