Easter eggs

According to the English Wikipedia, a virtual Easter egg is an intentional hidden message, inside joke, or feature in a work such as a computer program, web page, video game, television program, movie, book, or crossword. Minecraft features several easter eggs hidden within the game.

April Fools 2011
The Locked chest was a naturally generated block used as the premise of Notch's 2011 April Fool's Day joke. Upon attempting to open the chest, a message would appear stating that a Steve Co. Supply Crate Key is needed, and that one could be purchased at the Minecraft store. 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.

Halloween 2012
From October 30th-November 1st, 2012, Zombies, Skeletons, and other mobs would wear Pumpkins and Jack 'o' Lanterns on their heads.

Christmas 2012
From December 24th-26th, 2012, Chests and their larger counterpart had their textures changed to Xmas Chests. A texture known in the game files as "skis.png", was added in the Christmas/New Year update in the same folder as the Xmas Chests. Its textures were added by Dinnerbone as a red herring.

Zombie Pigman
In an unused are for the Zombie Pigman's texture, the words "THX XAPHOBIA" can be seen. This was written by Notch to give credit to the texture's original creator, XaPhobia.

MissingNo.
If one were to alter the file splash.txt without deleting the META-INF folder, the splash text on the title screen would appear to be missingno. This is a reference to the infamous glitch Pokémon MissingNo.

Minceraft
There used to be a one in 10,000 chance that the title screen will display "Minceraft" instead of "Minecraft". Though the logo change no longer exists, at the end of a crash log, the phrase "You should try our sister game, Minceraft!" can be found.