Start the game with this line added to the game's executable, +set developer +set ui_console 1 +set cheats 1 +set thereisnomonkey 1 +set cl_playintro 0.

You can now press TILDE key to display the console window and enter cheats. You change +set cl_playintro 0 to +set cl_playintro 1 if you want the introduction sequence.

Note: Cheats not available in v1.11 and higher.

God mode ... dog
Full health ... fullheal -or- fullheald
All weapons/ammo ... wuss
No clipping ... noclip
Invisible ... notarget
Third person view ... cg_3rd_person
List inventory ... listinventory
Teleport to location ... tele
Level select ... map