The 11^th Hour

Advance in game:
Go to the skull load-in screen. Type the following case-sensitive text, with spaces as noted: Roberto Dan R Zaphod B [Space]

After you type Roberto, Stauf will say, " Duh.... that's not it Karl!". Ignore him, he is trying to trick you. After the B, type [Space], then any of the following letters to get to that point in the game:

A: End of disc one.
S: End of disc two.
D: End of disc three.
F: End of disc four.
E: The "Olive In Stein" riddle, good for using the Kellyn code.
G: The very end, at the "Let's Make A Real Deal" intermission sequence.

Ending bonus:
Successfully complete the game to unlock the "Open House" file. Load it to be able to play any puzzle an unlimited number of times.

Kaleidoscope screensaver:
Start the game and go anywhere in the mansion. Remain idle and eventually the colors on the screen at that time will blend together as a screensaver. It is different from every viewpoint in the mansion.
Information in this section was contributed by Coolgamer167.

Hidden video:
Go to the television In Dutton's room at a point where you can see the close-up view. Type Kellyn to see a hidden video.
Information in this section was contributed by Coolgamer167.

Mocking cheaters:
In The 7th Guest, typing Zaphod Beeblebrox gave you open house access. Doing this in The 11th Hour will only cause Stuaf to mock you.
Information in this section was contributed by Coolgamer167.

Myst reference:
Get to a point in the game where you have access to the altar room. On the side here, is a bowl you can click. There is a page sticking out among the bones that says "Missed". This is an obvious reference to Myst and pages, the game that the 7th Guest was accused of trying to mimic.
Information in this section was contributed by Coolgamer167.