Hold Shift, choose the "Developer" tab and check "Enable Developer Mode" to continue loading the game. Then, hold Ctrl and click on "New Game" to set a difficulty setting and location. While playing the game, press \ to display the talk feature and enter the codes followed by Enter:
God mode: iamgod
God mode in last stage: iamgoddess
Full health: smoghead
Kill nearby enemies: giants
Unlock doors: opensezme
Switch view to nearest enemy: switch
Remove fog: foghack
Toggle flight and no clipping modes: fly
Land with flight and no clipping modes active: drop
Enable debug mode: debug on
Disable debug mode: debug off
List weapon names in debug mode: all weapons
Get indicated weapon: give [weapon name]
Get indicated item: gimme [item name]
Change speed: fov [6-93]
Set game speed: speed [1-50]