Locked chest

Locked chests (also known as Steve Co. Supply Crates) were a naturally generated block used as the premise of Notch's 2011 April Fool's Day joke. On April 5, they were made to disappear from worlds and as such are no longer obtainable legitimately. Since then locked chests have been stripped of their texture and are now a placeholder block.

Functionality
Every chunk generated between April 1 and April 5, 2011 had a small chance of having a locked chest in it. Clicking on the chest would bring up a screen, and clicking on the "Go to Store" button would forward you to the "Minecraft Store", an April Fools' Day joke, which has now been removed. The chest emitted a light level 15 and, before their removal from natural generation, was indestructible by hand or with a tool, but could be removed by explosions (which had a chance to drop them as an item, allowing them to be picked up).

Notch planned to remove them at midnight April 1, but didn't due to illness. Instead they were removed in the Beta 1.4_01 patch on April 5th. They were not removed from the game entirely; instead code was added for locked chests in the world to disappear, causing them to decay in a fashion similar to leaves. If locked chests are stored in the inventory, they remain until they are placed in the world. As of snapshot 12w21a, the old chest textures were removed from the game. This caused the Locked Chest to take on the blank purple texture.

Items "sold"
Eight "items" were "available": As well as three "Coming Soon" items (Priced at $75, $7, and $1499) and a "Regular Padlock" for $8.
 * Steve Co. Supply Crate Key ($3)
 * Steve Co. Supply Crate Key 5-Pack ($18)
 * Miner's Helmet ($21)
 * Name Change ($99)
 * Creeper Saddle ($42)
 * Secret Griefer Identity ($155)
 * Enlargement Pill ($2)
 * Response From Support ($494)

Trivia

 * The Beta 1.4_01 update made locked chests destroyable with one hit. If not destroyed, they would despawn, like leaves.
 * Locked chests are a reference to the Mann Co. Supply Crates 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.
 * The store page uses the font tf2build.ttf from Team Fortress 2. The font is downloadable from here.
 * Clicking on the proceed to checkout button triggers a screamer in which a Velociraptor from the movie Jurassic Park will growl loudly and fly across the screen.
 * Entering items totaling over $10,000 into the cart causes a window pop-up asking if you are prone to seizures. Entering OK makes the screen start flashing. When first released, there was no seizure warning label, Notch later implemented it to avoid lawsuits and posts on the forum.
 * This may have been what Notch was talking about in his Twitter post about a "silly secret project ."
 * Locked chests could generate in mid air or in the tops of trees.
 * If destroyed by TNT and collected, locked chests can be used as fuel in a Furnace like normal chests. They burn for 15 seconds each.
 * When placed on ice, locked chests act like torches and melt the ice around them. When placed in groups, they will "melt" each other until each chest resides on its own block of ice separated from others.
 * Locked chests could be placed next to each other to form a 2 block long chest like normal chests do. These two chests are still however "locked".
 * On April 2nd (the day following April Fools), the store had the addition of a multicolored banner which read '♥ April Fools! ♥', accompanied by a midi instrumental of Rick Astley's 'Never Gonna Give You Up'. Tobias Mollstam stated that this was "the best script he's written to date ."
 * Any locked chests previously loaded (naturally) will be gone. This means that if you found a chest after Beta 1.4, but before Beta 1.4_01, it will be gone. Any locked chests that were placed by players, such as those on multiplayer servers, have no function, other than providing light, and if left clicked will just despawn.
 * The server console outputted "added a chest!" each time a chest was generated.
 * Even though the regular Chest's model was changed in Beta 1.8, the locked chest still uses the old model.
 * Ocelots and tamed Cats will not sit on locked chests, unlike their behavior with regular chests
 * The 5-key pack is actually $3 more than 5 single keys.
 * The page is available at a reduced functionality here.