Press "\" to display the talk feature, then type the desired
code and press enter:

God mode -- iamgod
Full health -- smoghead
Enable debug mode -- debug on
Disable debug mode -- debug off
God mode in last stage -- iamgoddess
Kill nearby enemies -- giants
Land with flight mode -- active drop
List weapon names in debug mode -- all weapons
Obtain indicated item -- gimme ****
Remove fog -- foghack
Switch view to nearest enemy -- switch
Toggle flight mode -- enabled fly
Unlock doors -- opensezme

Developer mode:

Hold down Shift while starting the game.
Go to the "Developer Mode" tab and select the options you want.