The Ice Key is a normally uncollectable item from the 1998 N64 game, Banjo-Kazooie, found inside Wozza's cave in the Freezeezy Peak level. It can only be aquired in-game by entering a secret code (not revealed in-game) on the sandcastle floor, or via Gameshark. Collecting this item brings enables the mysterious, often speculated about, and ultimately useless "Stop-n-swop" menu in the game.
The Ice Key made a smaller and less legendary return in the sequel, Banjo-Tooie.
It is also related to the colorful mystery eggs.