An easter egg. A tribute. An in-joke.
Whatever you want to call them, the secret costumes that are available in Fatal Frame IV got us thinking again about the sly nods to other titles and secret touches that appear in so many games. You know the sort of thing we mean: the Chris Houlihan room in The Legend of Zelda: A Link to the Past. The Mount Rushmore trick in Pilotwings 64. The pictures of Mario, Luigi, and Bowser that hang in Hyrule Castle. We love this stuff, partially because we're shameless nerds, but also because it serves as a kind of in-joke between the developer and more knowing players.
Tons of games contain these secrets, but what's your absolute favorite videogame easter egg? Try and keep it to Wii games (or at least Nintendo software), and avoid the obvious (such as Yoshi's appearance in Super Mario 64) if possible. The more obscure, the better!