The thread about StarTropics got me thinking again: What games absolutely needed the manual or game cover to complete the game? I'm not talking about copy protection, either, although that would certainly apply. I can think of a couple:
The tunnel code in StarTropics, already mentioned.
In Metal Gear Solid, the back cover contains a codec number that you must use in the game to contact a specific person. The codec number is not revealed in-game, either.
Anybody else?