When I was nine or so, I was playing Duke Nukem on N64. I found that room with the drawing on the wall of the woman with the tentacle monsters. It was horrible Q_Q /childhood-destroyed
Hmm... But other than that, I can't recall any "secret" areas in games that I've found or looked for. I had the OoT Guide and I found a lot of Heart Containers and all four bottles, but I don't know if that really counts since it was kinda part of the game to begin with.