Ice Key

The Ice Key, as its name implies, is a large key made of ice. It is one of the items in the Stop 'n' Swop mystery, first being found in Banjo-Kazooie, and again in Banjo-Tooie where it can be utilized to unlock a secret transformation. There is also an unused text in Donkey Kong 64, implying that it was planned to be used at some point in the game.

Banjo-Kazooie
In Banjo-Kazooie, the Ice Key is found in Wozza's Cave in a large alcove, although the room is blocked by an unbreakable transparent wall of ice. Entering the code "CHEAT NOW YOU CAN SEE A NICE ICE KEY WHICH YOU CAN HAVE FOR FREE" on the Sandcastle floor in Treasure Trove Cove will dissolve the wall, allowing the player to access the key. However, as with the other Stop 'n' Swop items, it has no apparent function in Banjo-Kazooie.

In the Xbox Live Arcade version, entering the cheat will cause it to stop saving and prevent your scores being loaded to the leaderboard. However, having a Banjo-Tooie (XBLA) or Nuts &amp; Bolts save file will make it accessible.

Banjo-Tooie
In Banjo-Tooie, it is possible to obtain the Ice Key without the use of cheats. In the Jinjo Village, just to the right of the entrance, there is a small icy cavern which can be accessed using the Grip Grab ability. There is a Banjo-Kazooie Game Pak inside which will yield the precious Ice Key when it is destroyed. However, in the Xbox Live Arcade version, the Banjo-Kazooie Game Pak contains the Gold Mystery Egg and the Ice Key will automatically be added to your collected Stop 'n' Swop items if it was obtained in Banjo-Kazooie (XBLA).

Take the Ice Key to the Waterfall Cavern in Glitter Gulch Mine and use the Talon Torpedo learned from Jolly Roger's Lagoon on the Kazooie boulder in the water. Swim through the tunnel that opens up to a large room on the Icy Side of Hailfire Peaks containing a gigantic ice safe. The key can be used to unlock the safe, revealing the powerful Mega-Glowbo which can be brought to Humba Wumba in the Pine Grove to transform Kazooie into the fearsome Dragon Kazooie.

Banjo-Kazooie: Nuts & Bolts
In Banjo-Kazooie: Nuts and Bolts, if the Ice Key is collected in Banjo-Kazooie (XBLA) it will activate a crate on the roof of Boggy's Gym. This crate gives you the British flag.

The Ice Key can also be seen in Banjoland stuck in the melted Snowman and in the Terrarium of Terror where it must be collected as a Jinjo Fetch item. The Jinjo says he will "swop" the Ice Key for a Jinjo Token. After receiving the Ice Key, he says that he will ponder over the mystery of it. There is also a picture of Banjo and Kazooie holding it as well as the Pink Mystery egg.

Donkey Kong 64
In the text of the Rareware/Nintendo game Donkey Kong 64 from 1999, there was an unused line that read "ICE KEY", suggesting that at some point it was meant to be used in the game, and although the item was removed, the text remains. There are also videos on youtube in which people have modified the emulators to show where the Ice Key would have been, and how it would have appeared on the pause screen. It's intended use is unknown, but much like the Fungus Forest/Fungi Forest trade between the games, it's just another mystery shrouded in Rare's offices.