Locked chest

The Locked chest or Steve Co. Supply Crate was a block intended to direct players to the Minecraft Store, a site created as an April Fools' gag.

Breaking
When first introduced in Beta 1.4, locked chests were indestructible except to explosions, which allowed it to drop.

From Beta 1.4_01 up until its removal, locked chests were broken the same as normal chests.

Natural generation
In Beta 1.4, if the date is April 1st of any year, the game picks a random location to generate a locked chest in every new chunk. If the chosen location is air and has a solid block below, a locked chest will generate.

Usage
The locked chest's initial purpose was to direct players to the Minecraft Store. Clicking the would result in the dialog box to the left being displayed.

This functionality was exclusive to Beta 1.4; all later versions did not trigger this.

Placement and orientation
Locked chests would decide their facing direction according to the blocks around them, generally making an attempt to face outwards from walls. If their face was obstructed by a block, they would rotate accordingly to face away from said block.

Prior to Beta 1.8, said behaviour was standard for directional blocks such as chests and furnaces. From Beta 1.8 onwards, said blocks would instead face toward the player placing them, and would not change their facing direction depending on nearby blocks. The locked chest, being an unsupported block, was not subject to this change, and retained its old behaviour up until its removal, making it unique in this aspect.

Fuel
Locked chests could be used as fuel, lasting 1.5 items, the same as a regular chest.

Trivia

 * Steve Co. Supply Crate is a reference to the Mann Co. Supply Crate from Team Fortress 2, where a key must be purchased for $2.49 in the in-game shop called Mann Co. Store in order to open the crate.
 * This may have been what Notch was talking about in his Twitter post about a "silly secret project".
 * At the time of the locked chest being added into Minecraft, it looked identical to a normal chest. However, after Beta 1.8, when the chest model was updated to be smaller than a full block, the locked chest, a deprecated block at that point, remained with the old model, and as such became larger than an ordinary chest.
 * Unlike normal chests, locked chest luminates the world with a light level of 15. This was probably done to allow players to differentiate itself with normal chests.