Run the game with the "+set ui_console 1 +set cheats 1" command line. During the game, hit tilde (~) to bring down the console, then enter any of the following codes:

Result - Code

God mode - god
Freeze the game - game freeze
Have all weapons - give all
Health at 100 - health 100
Invisible - notarget
List maps - maplist
Demo game - demo
List models - modellist
List skins - skinlist
No clipping - noclip
Play game music - playsong
Quit game - quit
Reset game - reset
Spawn weapon - give [weapon name]
Record game - record
Stop recording game - stoprecord
Show shaders - shaderlist
Show memory info - meminfo
Show system info - systeminfo
Show inventory - inventory