Red Winter Hat

The Red Winter Hat was a novelty hat, but was changed to Cloth armor as of Patch 2.3.

Source
During the Feast of Winter Veil, the Red Winter Hat is a very rare world drop for humanoid mobs.

At the same time, the hat is also a boss drop for specific mobs based on which expansions the player owns.

For those that own only the basic game, the hat drops from:
 * Emperor Dagran Thaurissan in Blackrock Depths
 * Cannon Master Willey in Stratholme
 * War Master Voone in Lower Blackrock Spire

For those that own The Burning Crusade, the boss drops change to:
 * Captain Skarloc in Escape from Durnholde Keep
 * Nethermancer Sepethrea in The Mechanar
 * High Botanist Freywinn in The Botanica

For those that also own Wrath of the Lich King, the boss drops change to:
 * Grand Magus Telestra in The Nexus
 * Jedoga Shadowseeker in Ahn'kahet

For those that also own Cataclysm, the boss drops change to:
 * High Priestess Azil in Stonecore
 * Corla, Herald of Twilight in Blackrock Caverns

In 2007 the hat dropped from all six Azeroth and Outland bosses regardless of expansions owned.

In 2010 the hat dropped from Grand Magus Telestra and Jedoga Shadowseeker even for players who owned Cataclysm. Based on reports on Wowhead, and from personal experience, the drop rate is 100% for bosses in Heroic dungeons, but much lower for the same bosses in non-Heroic dungeons. Note that the hat is a normal loot item, so only one player will get it.