Who is Chris Houlihan, you might ask (if you aren't, as we established one potential reader may be, Chris Houlihan)? In 1990, Nintendo Power held a contest in which the winner's name would appear in an upcoming Zelda game. Chances are, Chris never actually saw the resulting room in A Link to the Past with his name in it, because it was basically designed never to be seen. Which is a really cool prize as far as we're concerned. In a way, Chris also received a decent amount of prize money. Unfortunately, it's all in Rupees.
The Chris Houlihan room, as it's called, contains 42 blue Rupees and a plaque that reads "My name is Chris Houlihan. This is my top secret room. Keep it between us, OK?" When you leave, you come out of Link's uncle's house. The room is basically a friendly error message, occurring when the game has yet to load the correct room for a certain location. As such, it requires some ingenuity to enter. The linked FAQ provides the method, as does this video, which also has the benefit of demonstrating the secret's accessibility in the Virtual Console version of A Link to the Past. The easiest and most well-known method is to rush from Sanctuary to the hole outside Hyrule Castle used at the beginning of the game. If you get there quickly enough, you'll fall into Houlihan's Room.
The room is significantly harder to access in the Game Boy Advance release of the game. The glitches that usually lead to the room have been fixed, leaving only cheat devices capable of granting entry. Furthermore, the tile announcing Chris Houlihan's name and secret message was removed, leaving nothing but a lousy, inaccessible room full of money. Even the nice blue water was taken out.