At the main menu, choose "Options", then select the "Keyboard" tab, and choose "Advanced". Place a check next to "Enable Developer's Console", then click "OK" or "Apply". While playing the game, press ~ to display the console window. Type "sv_cheats 1" and press [Enter] to enable cheat mode. Then, type one of the following codes at the console window and press [Enter] to activate the corresponding cheat.

Effect	Code
All weapons	giveall
Enemies ignore you	notarget
Flight mode	noclip
God mode	god
Never die	buddha
Normal speed	host_timescale 1
Slow motion	host_timescale 0.1