9. Chris Houlihan's Room - Legend Of Zelda: A Link To The Past
A rather famous secret, yet one worthy of discussion nonetheless. In 1990, Nintendo Power held a contest, in which a randomly selected participant would have their name featured and coded into an upcoming NES title. The winner was the titular Chris Houlihan, and the secret of his room wouldn't be discovered until 2000.
Essentially, the room was considering a fail-safe. The player would be teleported to this room if the game couldn't figure out which room they were meant to be in, and thus it was never intended to be a part of the actual gameplay. The only way to access it was the trick the game into breaking and sending you to the room.
It took a fair while for the community to discover this and it took a decade for Chris Houlihan to be given the credit for his winning of the contest. As a result, his name is one of the most famous secrets in Legend of Zelda. A damned good reward if anything.