Eye of Ender

An Eye of Ender is a craftable item the player uses to activate an End Portal. This is done by placing them into the End Portal Frame. It is also used to find the portal via leading the player to an End Portal Frame inside of a Stronghold.

When thrown, it will float diagonally upwards for about fifteen blocks in the direction of the nearest Ender Portal Frame, emitting 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 particles and disappearing. When the player throws it within twenty blocks of the Stronghold's portal, it will float toward the ground directly above the portal.

Players travelling 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 a dozen Eyes will be needed to activate it.

Also when thrown they will go through solid blocks but can be retrieved if you break the block it ended floating in.

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 home 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.