Debug Mode  At the password screen, type QQQQ QQQQ QQQQ QQQQ until you get an error message. Then, go to the options screen. (Thanks to SOUTHaPARC@aol.com, ShopLMart@aol.com)

Nightmare Difficulty  Enter the slipgate on any difficulty level. In the episode 4 room, fall in the water, head under the ledge and stand there. When falling through the water, don't move. You'll land on a platform. Then, walk across the platform and enter the Nightmare slipgate. 

No Clipping  Password: NOCLIP. (Thanks to LWargalla@t-online.de)