Eye of Ender

The Eye of Ender (or Ender Eye) is a craftable item most notably used to activate an End Portal. Eyes can be used to activate End Portals by being placed into empty End Portal Frames until the entire ring of 12 Frames are filled, thereby activating the End Portal. They are also used as an aid in finding the End Portal by leading the player to an End Portal Frame inside of a Stronghold when thrown.

When thrown, it will float diagonally upwards for about fifteen blocks in the direction of the nearest Ender Portal Frame, traveling through any type of block, leaving a trail of purple particles. It will float in the air for three seconds before either falling back to the ground as a collectable item or shattering into pieces and disappearing. When thrown within twenty blocks of a Stronghold's portal, it will float toward the ground directly above the portal.

Players traveling to The End are advised to craft multiple Eyes of Ender since each one has a chance of disappearing after being used to locate the Stronghold containing the portal and up to 12 Eyes will be needed to activate it.

History
The Eye of Ender was first added as an item in Beta 1.9 Pre-release 3, but had no use until Beta 1.9 Pre-release 4, when each Eye could be placed in a portal block or used to hone in on a Stronghold. Jeb demonstrated the new uses for an Eye in his livestream. An End Portal within a Stronghold could be seen in the stream with two Eyes inserted into blocks.

Trivia

 * Eyes of Ender were oddly large from third person or other players view in multiplayer. It also appeared to be held at a 90 degree angle from the original orientation. This is due to a programming error, which specifies that Eyes of Ender should be wielded as a "tool" in third person. This error was shared by Sugar. As of the Full Release this has been fixed.
 * The Eye of Ender's right-click function only works in the Overworld and in Default Mode (Default mode as of 1.1).
 * In 1.9 Pre-release 5, there is a bug when, if the eye is thrown, it will make both the arrow shooting sound and the egg throwing sound.
 * In older worlds, generated before 1.9 Pre-release 3, the eyes may mislead you to a place where there isn't a Stronghold at all. This is most likely because the eyes lead to where a Stronghold should be based on the seed, and seeds generate worlds differently in older versions. Therefore, if you saved the coordinates the eye traveled to in an old world and generated a new world with the same seed, you could travel to those same coordinates and find a stronghold.
 * Likewise, if the world is generated without the "Generate structures" option enabled, the Eye of Ender will lead you to the place where the Stronghold would have been generated.
 * As Strongholds are not generated in the Superflat world type, you cannot throw the Eye of Ender there.